XPS Study of
Hydrodesulfurization Catalyst
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 889
An X-Ray Photoelectron Spectroscopy Study of Various CoMo/Al,O, Hydrodesulfurization Catalysts R. I. Decierck-Grimee, P. Canesson,* R. M. Friedman,+ and J. J. Fripiat Groupe de Physico-Chimie Min6rale et de Catalyse, Universit6 Catholique de Louvain, 1, Place Croix du Sud, B 1348, Louvain la Neuve, Belgium (Received October 1, 1976; Revised Manuscript Received February 6, 1978) Publication costs assisted by Universit6 Catholique de Louvain
Various series of CoMo/A1203 hydrodesulfurization catalysts have been studied by x-ray photoelectron spectroscopy (XPS). After sulfiding, practically all the molybdenum is in the MoSzform, but two cobalt species can be observed. Some of the Co2+ ions are in an oxygen environment (Co-I)probably as a pseudo-aluminate; the remainder appear as sulfided cobalt (Co-F),probably Cogss. The catalytic activity can be related to the ratio (Co-F)/Mo.
Introduction Current concern over sulfur-containing emissions from combustion of fossil fuels has focussed attention on CoMo/A1203hydrodesulfurization (HDS) catalysts. In spite of their importance and although it is generally acknowledged that the activity for desulfurization rests essentially on species containing, in majority or exclusively, molybdenum, these catalysts are poorly understood, as evidenced by the lack of agreement on the role played by cobalt ions as pr~moters.l-~ Since the behavior of cobalt under sulfiding conditions is dependent upon the presence or absence of molybdenum on the surface+ we have investigated several series of both experimental and commercial CoMo/A1203catalysts. XPS has been used to investigate the chemical state of the surface species and to measure the relative abundance of the different forms in which a given element is present. Our findings are discussed in relation to the variation of catalytic activity, with consideration of possible interactions between the active phases and promoters. Experimental Section (1)Material. Four series of CoMo/A1203catalysts have been studied. They are denoted as A, B, C, and D series. Series A and B are laboratory prepared catalysts. They are obtained by a minimum solution impregnation of the same y-alumina (surface area 169 m2 g-l) with cobalt nitrate and ammonium paramolybdate solutions. For series A catalysts, molybdenum was first impregnated; the catalyst was subsequently calcined for 16 h at 500 "C. Various amounts of cobalt ions were deposited in a second impregnation step followed by a final calcination at either 500 or 700 "C for 16 h. The Moo3content was held constant, at about 14 wt % Moo3 for AI to A7 catalysts and at about 22 wt % for As to Alp The COO weight percent varies in such a manner that the Co/Mo atomic ratio ranges between 0.13 and 1.93. In series B, about 4.5 wt % of COO was first impregnated and the resulting material was calcined at 500 or 700 "C for 16 h. Various amounts of molybdenum were then deposited; the final calcination was made at 500 "C. The atomic Co/Mo ratio varies from 0.43 to 1.43. The principal characteristics of these two series are summarized in Table I. 'Present address: C.N.R.S., C.R.S.O.C.I., lB, Rue de la FBrollerie, 45045 Orleans, Cedex, France.
'Permanent address: Corporate Research Department, Monsanto, 800 N. Lindberg Boulevard, St. Louis, Mo. 63166. 0022-3654/78/2082-0889$01 .OO/O
The C series consists of 14 commercial catalysts from various manufacturers. Table I1 gives the principal characteristics for this series. Series D catalysts have been synthesized in the laboratory according to various preparation methods. The principal characteristics and the details concerning the preparation methods are summarized in Table 111. (2) Catalytic Activities. All the determinations have been made on an Iraqi gas oil containing approximately 1w t % sulfur, For the A, B, and D series, catalytic activity measurements have been made in a microreactor. The activities are expressed as the LHSV (liquid hourly space velocity expressed as the volume of liquid feed injected through the unit volume of catalyst per hour) needed to obtain a 30% conversion and are relative to a value of 100 arbitrarily assigned to the A3 catalyst. For series C catalysts, catalytic activities have been obtained in an industrial fixed bed reactor; they are also defined as the LHSV needed to obtain 30% conversion and are relative to an arbitrary value of 100 given to C5 catalyst. Results are given relative to the unity of mass of catalyst (relative specific activity) and relative to a surface of 100 m2 of catalyst (relative intrinsic activity). (3) XPS Measurements. They are performed using a Vacuum Generators ESCA 2 system. A detailed description is given el~ewhere.~ The peak intensities were measured by planimetry of the graphic displays of the spectra. All the XPS studies have been made after the samples have been sulfided at 400 "C using a 15-85% mixture of H2S-Hp4v5
Results and Discussion (1)Binding Energies. The position of the XPS lines observed after the sulfiding treatment are identical for all catalysts studied. Binding energies, relative to the A1 2s line of the carrier which was arbitrarily positioned at 118.0 eV, are summarized in Table IV. They are identical with those found for some industrial catalysts after the same treatment (a) Cobalt. Figure 1 shows clearly that at least two cobalt species exist in HDS catalysts. The Co 2p3/, spectrum has been decomposed assuming that the lower binding energy line, denoted Co-F, has a full width at half-maximum (fwhm) of 2.9 eV and the principal higher binding energy line (denoted Co-I) has a fwhm value of 3.9 eV. The fwhm used for each individual peak to decompose the composite Co 2p3I2spectra was chosen from 0 1978 American Chemical Society
890
Canesson et al.
The Journal of Physical Chemistry, VoL 82, No. 8, 1978
TABLE I: Principal Characteristics for Catalysts of Series A and Ba
A1 0
14.9 13.4 14.7 13.0 14.3 15.3 14.5 21.8 23.4 21.4
2.3 3.1 4.4 7.0 8.1 14.6 3.8 8.3 12.7
Surface area, m1 g-l 152 153 151 148 134 I35 110 121 106 103
A6-700
13.4 13.3 15.3
2.2 4.5 8.5
104 131 110
37 54 59
36 41 54
0.114 0.223 0.491
0.180 0.378 0.643
0.63 0.59 0.76
6.3 11.6 15.0 19.4
4.7 4.8 4.7 4.4
142 142 130 125
27 38 36 31
19 27 28 25
0.376 0.422 0.386 0.368
0.251 0.219 0.241 0.219
1.50 1.93 1.60 1.68
Wt % Catalyst A, A2 A3 A4 A, A6
A7 A, A9
Bl B2 B3 B4
MOO,
wt %
coo
1.0
Re1 specific activity 46 51 100 78 60 49 58 71 67 39
Re1 intrinsic activity 30 33 66 53 45 36 67 59 63 38
Co-F, mmol g-' 0.083 0.180 0.255 0.361 0.562 0.689 1.227 0.306 0.736 1.110
co-I, mmol g-l 0.050 0.127 0.159 0.226 0.372 0.392 0.721 0.201 0.372 0.585
Co-F/
co-I 1.66 1.42 1.60 1.60 1.51 1.76 1.70 1.52 1.98 1.90
4.7 133 6.3 32 24 0.231 0.396 0.58 4.9 126 38 30 0.309 0.345 0.89 16.7 a Series A: Mo always deposited first; intermediate calcination at 500 'C; final calcination at 500 ' C for A, to A,, and at 700 C for A 2 , 4 a- 760 0 . Series B: Co always deposited first; intermediate calcination at 500 'C for B, to B4 and at 700 O C for B1,3.700; find calcination at 500 "C. B1-700
B3-700
TABLE 11: Principal Characteristics for Series C Catalysts
Catalyst
Wt % MOO, 12.3 12.6 12.4 11.1 11.7 12.2 12.8 15.1 13.5 12.8 14.7 12.8 11.2 11.7
Surface Wt % COO area, m 2g-' 4.5 331 311 4.2 4.8 320 3.7 298 3.7 303 3.7 300 2.8 195 3.5 281 2.8 232 345 3.1 221 2.4 285 2.9 248 3.3 311 3.1
cases where that component dominated the spectra. The width of other well-resolved peaks, e.g., A1 2s,O Is, or Mo 3d, were compared from one sample to another to assure that grossly different charge dispersions were not misleading. It is important to note that there is no satellite line associated with the Co 2p312-F peak but a significant one with the Co 2p3/,-I peak. This well-known satellite is related to a shake-up phenomenoq6 it is characteristic of paramagnetic Co2+ ions in a high spin environment. Because of its origin which removes intensity from the main peak, the satellite line intensity must be included with the Co-I species in any consideration of relative abundance of the different species. The binding energy corresponding to the Co-F species is different from the value found for It corresponds to the binding energy of Co2+in a sulfur environment, as in pure Cogss. The Co 2p3/, line of pure cogs8 is illustrated on Figure 2. In order to compare with HDS catalysts the bindhg energy scale for this compound is referenced to a thin evaporate film of gold on the surface. The Au 4f712 line is positioned at 83.0 eV. The principal feature of this Co 2~312line is the absence of any satellite when it exhibits a very important one in COS.^ These results suggest that the Co-F species in HDS catalysts are quite similar to
Re1 specific activity 108 102 102 102 100 95 93 92 86 84 65 61 59 43
Re1 intrinsic activity 33 33 32 34 33 32 48 33 37 24 29 21 24 14
CO-F, mmol g-' 0.209 0.183 0.234 0.148 0.158 0.191 0.218 0.161 0.205 0.133 0.168 0.144 0.277 0.168
co-I, mmol g1 0.392 0.378 O.PO7 0.346 0.336 0.303 0.156 0.306 0.169 0.281 0.152 0.243 0.163 0.246
1
Flgure 1. XPS profile of the Co 2p3,, level for C, catalyst after sulfklation.
cobalt in a bulk cogs8 phase; in the catalysts, the cogs8 would be finely dispersed on the support. The remaining cobalt, denoted species Co-I, is not sensitive to the sulfiding treatment. As in the case of Co/A1203 catalyst^,^ one can conclude that the corresponding cobalt ions are probably bound at tetrahedral
XPS Study of Hydrodesulfurization Catalyst
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 891
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The Journal of Physical Chemistry, Vol. 82, No. 8, 1978
Canesson et
81.
TABLE IV: Position of the Various Levels after Sulfiding Binding energy, eV
161.3
I
775
228.2
718.3
781.2
785.9
I
780
785
eV
Figure 2. XPS profile of the Co 2p3,, level for pure Co&.
sites of the carrier. The pseudo-aluminateformed this way is insensitive to reducing and sulfiding under the conditions where these treatments have been made. (b) Molybdenum. For all the catalysts studied, as for pure Mo/A103, the binding energy values of the Mo 3d levels after treatment with H2-H2S are very close to those found in pure MoS2. There are no additional peaks which would correspond to unreduced molybdenum ions in an oxide environment. This fact disagrees with results previously published,8 probably because our samples have been studied without any contact with air or moisture between the sulfiding treatment and the introduction in the XPS equipment. Since a treatment of 20 h at 400 OC is a necessary condition in order to obtain a steady state for the surface composition, our results differ from those obtained by other author^,^ who observe an incomplete sulfiding of molybdenum species after a treatment of 10 h at 400 "C under a H2-H2S mixture flow gas. (2) Intensities. Assuming that all the cobalt ions are accessible to XPS analysis, it should be possible to calculate, at least to a first approximation, the quantity of Co-F and Co-I species present in the catalyst. This assumption seems reasonable for catalysts with a low cobalt content since the calcination temperature is not too high and it has been shown that the Co 2p,/, response is linear with the cobalt content for Co/A1203catalysts until all the surface is covered by cobalt ions.l0 In the catalysts studied here, the Co-F/Co-I ratio varies from 0.21 to 1.98. (a) Factors Influencing the Co-F/Co-I Ratio. For catalysts obtained with the same alumina and prepared by the same method (series A and B), Figure 3 shows that the relation between the total amount of cobalt introduced per unit surface area and the quantity of Co-I species is linear. This observation explains our earlier conclusions correlating the questionable activity with the quantity of c0-1.~ Unfortunately, at that stage we lacked surface area data. The transformation to intrinsic activity (Le., per unit surface area) removes the common factor and demonstrates that the correlation previously noted is a secondary effect (vide infra). For the same calcination temperature
co-
F m-2 (los6m a l e
m-2)
Figure 3. Relation between the quantity of Co-I (cobalt in an oxygen environment) and the total amount of Co for catalysts of the A and B series and Co-AI,O,. Calcination temperature 500 OC: (e)A, to A,; (0) As to Ala; (H) B, to B,; (X) Co, to Co, (see ref 4). Calcination temperature 700 OC: (A)A; (A)B.
(500 or 700 "C), the relation is the same for the A and B series, showing that the order of impregnation has no influence on the proportion of Co2+ions associated with the carrier framework. An increase in the calcination temperature results in a larger portion of the cobalt bound to the alumina, as shown in Figure 3 by the lower values of the slope for catalysts fired at 700 "C. In Figure 3, we have also reported the values for Co/ A1203samples after sulfiding (catalysts studied in ref 4 and denoted Col to co4). The linear relationship also holds for these catalysts except for the apparent deviation at low cobalt content. When cobalt is present alone on the carrier, a greater part of Co2+cannot be reduced and forms a surface cobalt pseudo-aluminate. The accompanying presence of molybdenum seems to inhibit the interaction of Co2+with the carrier. The affinity of Moo3 for the alumina surface is well known'lJ2 and this can be understood as a competition between the two ions for the occupancy of tetrahedral sites of alumina. Simultaneous presence of both types of cobalt coupled with a linear increase in the quantity of Co-I with the total cobalt content suggest that the dispersion on the surface is not perfect. The deposited cobalt phase may have a tendency to form small aggregates scattered on the surface; a constant portion of these aggregates would interact with the alumina framework forming the pseudo-aluminate. This relation between the quantity of Co-I species and the total amount of cobalt is only found within a series of catalysts prepared in an identical way. The situation is different for catalysts of series C and D. In these catalysts, variable proportions of cobalt occupy the tetrahedral sites of alumina according to the preparation method and calcination temperature. Nevertheless, it can be concluded from these series that factors promoting the formation of a superficial aluminate are a high cobalt content up to a limiting capacity (e.g., compare Table I11 D1 and Dz or Dlo),
The Journal of Physical ChemWy, Vol. 82, No. 8, 1978 893
XPS Study of Hydrodesulfurization Catalyst
1
.7
.S
15
I 0.15
,
0.20
e11
Co-FJMo
Co-/Me
I
0.25
I
0.30
I
0.35
Figure 4. Relation between the intrinsic relative activity and Co-F/Mo ratio for C catalysts.
a high calcination temperature (compare Table I11 D5, Dg, and D7),and a carrier with a large surface area (compare D1 and D13). An increase in the surface area may promote the formation of cobalt aggregates of smaller size, and a higher ratio of cobalt is able to interact with the carrier, suggesting that dispersion of cobalt ions is not perfect. This is in marked contrast to recent studies of Cu/A1203 catalysts which show that the sites on the alumina become saturated before any appreciable agglomeration of a separate CuO phase occurs.13 We can only speculate whether this reflects the structural similarity of the Co304 and aluminas as spinels, and hence an expitaxial growth of the metal oxide, or a higher affinity of the Cu2+for the octahedral sites compared to Co2+for its tetrahedral location. (b) Relation with Catalytic Actiuity. On the basis of the above results, we have tried to correlate the superficial composition of the various catalyst series with the intrinsic catalytic activity. One can observe that there is no relation between this activity and the total amount of cobalt nor with the Co total/Mo ratio. Neither is a regular trend observed between the intrinsic activity and Co-F/Mo ratio. The maximum of this synergetic effect is obtained for a Co-F/Mo value of 0.2 for catalysts of the C series (commercial catalysts) and a value of 0.3 for laboratory prepared catalysts (A, B, and D series). The existence of these relations seem to indicate that the sulfided cobalt species has a promotor effect only on the active phase in hydrodesulfurization catalysts. The difference between commercial and experimental catalysts with respect to the Co-F/Mo value at the intrinsic activity maximum may be a consequence of a better dispersion of cobalt at the surface of the carrier in the C series where, consequently, a lower quantity of cobalt would be needed for attaining the maximum of activity. From Figures 4 and 5, it can also be noted that, for the same Co-F/Mo ratio, catalysts of series B have a lower activity than those of series A and D. In particular, catalyst A4 and B3have the same cobalt and molybdenum content but the intrinsic activity of A4 is twice that of B3 This fact may be related to the order of impregnation: for series B cobalt is deposited first while it is molybdenum which is first deposited for A catalysts. Good activity can thus be related to good accessibility of cobalt and molybdenum species and as well as a minimization of the interactions between cobalt and alumina. (c) Interpretation of the Synergistic Effect. Various models have been proposed to explain the promotion effect
Figure 5. Relation between the intrinsic relative activity and Co-F/Mo ratio for catalysts of the A, B, and D series: (e)A series calcination temperature 500 OC; (B) B series calcination temperature 500 OC; (A)A series calcination temperature 700 OC; (A)B series Calcination temperature 700 OC; (X) D series.
of cobalt ions on what is supposed to be active phase, Le., M o S ~ . Our ~ - XPS ~ ~ study ~ ~ indicates ~ ~ ~ that the relations shown in Figures 4 and 5 are of the same kind as those reported for nonsupported ~ a t a l y s t s .Whatever ~~ the active ions are, Mo3+2~14or special Mo species promoted by Co2+ ions in the carrier just under the molybdenum phase,' is must be observed that only sulfided cobalt species play a role in hydrodesulfurization, even if the genesis of each of these phases is not completely independent of the other metal.4 Sulfiding of the cobalt species deposited on alumina does not occur in the absence of m~lybdenum;~ the reduction of molybdic anhydride is easier when Moos is mechanically mixed with cobalt oxide or sulfide16which seems to activates hydrogen, but the addition of cobalt oxide or sulfide to Moo3 has no influence on its being sulfided and reduced to MoS2. From our experiments, it is not possible to make conclusions about the nature of the promotion effect. It may be an electronic transfer between molybdenum sulfide and cobalt sulfide at the particle interface. Another possibility may consist in a preferential activation by the cobalt phase of one of the components of the reaction mixture (i.e., HJ.
Acknowledgment. The authors thank Professor B. Delmon for helpful discussions and the revision of the manuscript. One of us (R.1.D.G) thanks I.R.S.I.A. for financial support. The provision of catalyst samples by Labofina is greatly appreciated.
References and Notes (1) V. H. J De Beer, T. H. M. Van Sint Fiet, G. H.A. M. Van Der Steen, A. C. Zwaga, and G. C. A. Schuit, J . Cab/., 35, 297 (1974). (2) A. L. Farragher and P. Cossee, Cafal. Proc. Inf. Congr. 5th, 1972, 1301 (1973). (3) G. Hagenbach, Ph. Courty, and B. Delmon, J. Cafal., 23,295 (1971). (4) R. I. Declerck-Grimee, P. Canesson, R. M. Frledman, and J. J. Fripiat, J . Phys. Chem., preceding article in this issue. (5) R. M. Friedman, R. I. Declerck-Grirnee, and J. J. Fripiat, J. Necfron Specfrosc., 5, 437 (1974). (6) C. S. Fadley and D. A. Shirley, Phys. Rev. A , 2, 1109 (1970). (7) D. C. Frost, C. A. McDowel, and I. S.Wmlsey, Mol. Phys., 27, 1473 (1974). (8) A. W. Amour, P. C. H. Mitchell, B. Folkesson, and R. Larson, J. Less Common Met., 38, 361 (1974). (9) T. A. Patterson, J. C. Carver, D. E. Leyden, and D. M. Hercules, J . Phys. Chem., 80, 1700 (1976). (10) J. Grimblot, J. P. Bonnelle, and J. P. Beaufils, J . Nectron Specfrosc.. 8, 437 (1976). (11) A. W. Miller, W. Atkinson, M. Barber, and P. Swlft, J. Cafal., 22, 140 (1970). (12) G. C. A. Schuit and B. C. Gates, AIChEJ., 19, 417 (1973).
094
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978
(13) R. M. Friedman, to be submitted for publication. (14) V. H. J. De Beer, T. H. M. Van Sint Fiet, J. F. Engelen, A. C. Van Haandel, M. W. J. Wolfs, C. H. Amgerg, and G. C. A. Schuit, J. Catal., 27, 357 (1972).
T. N. Rhodin, C. F. Brucker, and A. B. Anderson
(15) P. Canesson, 8. Delmon, G. Delvaux, P. Grange, and J. M. Zabala, 6th International Congress on Catalysis, London, 1976, Communication B 32. (16) J. M. Zabala, Ph.D. Thesis, Louvaln-la-Neuve, 1976, Belgium.
Structure and Bonding of Acetylene and Ethylene on a-Iron Surfaces at Low Temperaturest Thor N. Rhodin," Charles F. Brucker, School of Applied and Engineering Physics, Corneii University, Ithaca, New York 14853
and Alfred B. Anderson$ Chemistry Department, Yale Unlversity, New Haven, Connectlcut 06520 (Received June 24, 1977; Revised Manuscript Received January 5, 1978) Publication costs assisted by Corneii University
A combined UV photoemission and molecular orbital analysis is presented of chemisorption and reaction of acetylene and ethylene at low temperatures on a-Fe(100) iron surfaces. Both acetylene and ethylene appear to be dissociatively chemisorbed on Fe(100) at 98 K into CH, CHz,and other fragmented species. It is significant that the spectrum for fragmented ethylene is reproduced for hydrogen chemisorbed on a carbon-covered surface. It is proposed that a thin weakly chemisorbed molecular layer forms on top of the fragment-covered,more strongly chemisorbed surface. At higher temperatures, a gradual conversion of the molecular layer to the fragmented state occurs, involving the migration of loosely bound molecules to reactive surface sites and subsequent dissociative interaction. Reaction mechanisms are discussed to account for the facility of these reactions.
I. Introduction A combination of ultrahigh vacuum, low energy electron diffraction (LEED), and ultraviolet photoemission spectroscopy (UPS) has provided a unique combination of data for elucidating structures and bonding properties of chemisorbed and reacted species on iron metal surfaces. Effort is made in this paper to make a significant analysis based on quantum mechanical interpretations and bond energies and configurations. The reactive properties of iron toward acetylene, ethylene, and other small hydrocarbon molecules have been previously measured and de~cribed.l-~ A detailed molecular orbital analysis is made here to the interpretation of the chemisorption and reaction of acetylene and ethylene on iron in the 98-123-K temperature range. It is concluded that acetylene dissociatively chemisorbs primarily as CH fragments on an a-iron (100) surface and that ethylene dissociatively chemisorbes primarily as CH2 fragments. The role of other fragmented species is also discussed. 11. Experimental Section The experimental system and characterization of the clean Fe(100) surface and the corresponding UPS and LEED/MS results have been fully described previously.2t4 A four-grid LEED/AES spectrometer' modified for ups experiments by the addition of a
t Supported by the Materials Science Center of Cornell University, the American Iron and Steel Institute, and the National Science Foundation. t~epartmentof Chemistry, University of California-santa Barbara. 0022-3654/78/2082-0894$01 .OO/O
cold-cathode helium discharge light source giving monochromatic radiation at 21.2 eV, was used to perform the energy analysis. A computerized data acquisition and storage system with graphic display (DEC GT-40) greatly facilitated the accumulation of UPS spectra. Precise subtraction of spectra by this method made it possible to descriminate small but significant features in the spectral data, Difficulties in cleaning iron single crystal surfaces are due not only to the high chemical reactivity of iron but also because the lattice of the allotropic a form is stable only below 910 "C. An elaborate cleaning procedure was required to reduce the levels of the chief impurities (phosphorus, sulfur, carbon, nitrogen, and oxygen) to less than -3% of a monolayer total impurity c o n c e n t r a t i ~ n . ~ ~ ~ In the final stages of cleaning, involving the removal of residual carbon and oxygen by thermally activated surface titration, surface composition was monitored using UPS. Undesirable AES beam effects were thus avoided. In comparison with AES compositional analysis,UPS analysis can be more sensitive to trace oxygen impurity on Fe(100).2 111. Theoretical Procedure The theory used in this paper is an approximate but well-tested approach which combines atomic charge density and molecular orbital theoryS6 Rigid atoms are superimposed and the electrostatic two-body repulsion energies are using the H ~ ~ force ~ formula. The atomic electron charge densities are allowed to relax, hi^ is accomplished by using molecular orbitals and diagonilizingthe approximate molecular Hamiltonian which is a sum of atomic Fock potentials. It is then possible to use the new charge density to calculate the 0 1978 American Chemical Society
~
