9820
J. Phys. Chem. 1996, 100, 9820-9823
Modeling of Etching Reactions: Ab Initio Calculations of the Reactions of CFm+ (m ) 1-3) and NFn+ (n ) 1, 2) with Local Models of SiO2 Surface Structures Arndt Jenichen† Institute of Physical and Theoretical Chemistry, UniVersity Leipzig, Permoserstrasse 15, D-04318 Leipzig, Germany ReceiVed: NoVember 7, 1995; In Final Form: March 13, 1996X
Using ab initio calculations, reactions of CFm+ (m ) 1-3) and NFn+ (n ) 1, 2) with ≡SiOH groups and ≡SiOSi≡ bridges of SiO2 structures are investigated. The local surface structures are modeled by cutouts of the surface. Dangling bonds are saturated by hydrogen atoms. Reaction and activation enthalpies are estimated by applying the MP2/6-31G**//3-21G(*) procedure. According to the calculations, all the considered ions are able to form C-O and N-O bonds to ≡SiOH groups and ≡SiOSi≡ bridges (59-92 kcal/mol). NF+ and CF2+ have the strongest bonds with the O atoms. The existence of O-F, Si-C, and Si-F bonds could not be demonstrated. Starting from these structures the Si-O bond breaking is investigated. The direct dissociation of the Si-O bond neighboring the attacked O atom requires an energy of 30-45 kcal/mol. More favorable reactions go over cyclic transition structures with activation enthalpies in the range between 10 and 21 kcal/ mol. The reaction enthalpies show that under low-temperature conditions the surface is blocked by CF3+ and CF2+. Chemisorbed CF+, NF+, and NF2+ allow the break of ≡Si-OSi≡ bonds by cyclic rearrangement. The ≡Si-OH bond breaking is possible after chemisorption of NF2+ and probably NF+ and NF2+. Under high-temperature conditions as in discharges all the ions favor the break of Si-O bonds.
1. Introduction Plasma etching is an important method for surface processing where in the discharge a large number of different neutral as well as positively and negatively charged species are formed. Their different chemical behavior at the surfaces is largely unknown but important for optimization of the etching processes. The essential mechanism is the weakening and breaking of the bonds which connect the surface atoms with the surface. On the other hand, the surface can also be blocked by chemisorbed species. For etching of the Si/SiO2 systems, fluorocarbon and fluoronitrogen discharges are preferentially employed.1,2 Experiments demonstrated that the products of a discharge in NF3 can etch Si and SiO2 1-2 orders of magnitude faster than those of a similar discharge in CF4. Two reasons were found: A discharge is much more effective at dissociating NF3 than CF4 due to the bond strengths. No polymerization forming surfaceblocking films is found for NF3, contrary to pure fluorocarbon plasmas.3,4 Already in 1981, Ianno et al.4 pointed out that positive ions have the capability of contributing to the etch mechanism through chemical reactions. Bello et al.5 demonstrated that CF+ bombardment at 2 eV leads to molecular absorption with a small contribution of reactions with SiO2. A 20 eV bombardment causes strong reactions with SiO2. Hikosaka et al.6 found that CF3+ is the most abundant ion in a capacitively coupled plasma. This is different from inductively coupled plasma where CF+ ions form the majority. The last has a higher etch rate. In some previous papers,7,8 the Si-O bond breaking by neutral CFm (m ) 1-4) and NFn (n ) 1-3) was theoretically investigated. The main result that CF2 is the most efficient molecule for SiO2 etching is in accordance with the laser experiments. The reliability of the used method was demonstrated at the systems HF + Si9-11 and HF/HCl + SiO2.12 It is the aim of this paper to investigate the chemical reactions of CFm+ (m ) 1-3) and NFn+ (n ) 1, 2) with SiO2 surface † X
E-mail:
[email protected]. Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(95)03276-X CCC: $12.00
structures as ≡SiOH groups and ≡SiOSi≡ bridges. The main subjects dealt with the chemisorption of the ions and the Si-O bond breaking by cyclic rearrangements. In first part of the paper the local surface models and the used quantum chemical procedures are presented. In the second part the properties of the ions as well as their local chemisorption structures (LCS) and some possible surface reactions are discussed. 2. Models Ab initio calculations require the restriction to cutouts of the surface. Therefore, in this study only the neighboring atoms of the reaction center are considered. This model size should be sufficient because the aim of these investigations is to determine the differences in the reactivities of the various species. The influence of long-range interactions as well as steric constraints must be neglected. Furthermore, the numerous surroundings (other reactants, intermediates, and products) of the reaction centers (Si-O bonds) in a reaction layer cannot be considered by this model. Two reaction centers are regarded: ≡SiOH and ≡SiOSi≡. The surface ≡SiOH group is modeled by an orthosilicic acid (a). For the O bridges two models are used (b and c): HH H H HSi H Oat
Hip
Si
Oat
Oat
Sinb
OOop
Oip
HHop
Hip
a
OO O
Si
Sinb Oip
Hsi
OOop HHop b
Sinb Oip
OOop
Hip HHop c
The dangling bonds are saturated by hydrogen atoms.13 For model c, Cs symmetry is assumed. Equal atoms in different positions are marked by subscripts: ip ) in-plane, op ) out© 1996 American Chemical Society
Modeling of Etching Reactions
J. Phys. Chem., Vol. 100, No. 23, 1996 9821
TABLE 1: C/N-F Distances r (Å) and Net Atomic Charges q (e) (Mulliken Population Analysis) of the Individual Ions +
CF4 CF3+ CF2+ CF+ NF4+ NF3+ NF2+ NF+
r(C/N-F)
q(C/N)
q(F)
1.271/1.428 1.251 1.231 1.190 1.365 1.341 1.288 1.230
+1.54 +1.58 +1.34 +1.12 +0.99 +0.99 +1.06 +1.02
-0.23/-0.04 -0.19 -0.17 -0.12 0.00 0.00 -0.03 -0.02
of-plane, Si ) bonded at Si, at ) attacked atom, nb ) atom neighboring the attacked atom. Oat-Sinb determines the breaking bond. 3. Method The ab initio calculations were carried out with the GAUSSIAN 92 program system.14 The geometry optimizations (minima and saddle points) and the force constant calculation were done with the restricted and unrestricted Hartree-Fock method and the 3-21G(*) basis set.15 The gradients and force constants are calculated by analytical methods.16,17 IRC (intrinsic reaction coordinate)18 calculations performed for some reactions of model a show that the transition structures are connected with the correct reactants and products, analogous to ref 12. The zeropoint vibrational energies (ZPE) were corrected for known deficiencies of Hartree-Fock calculated frequencies with a scaling factor of 0.89.19 The energies of reactants, products, and transition structures were obtained using the 6-31G** basis set20 and the Møller-Plesset perturbation theory21 of second order (MP2/6-31G**//3-21G(*)) with frozen cores. 4. Result 4.1. Ions. Table 1 presents the C/N-F distances and the net atomic charges (Mulliken population) of the possible ions. Energy calculations for the lowest spin multiplicities show that the singlet or doublet states form the ground states. They are used for the following calculations. As expected, CF4+ is a complex composed of CF2+ and F2 and is, therefore, not considered in the following investigations. On the other hand, NF4+ and NF3+ are stable. The positive charge of the ions is localized at the C or N atom. For the NFn+ ions the charges of the F atoms are close to zero. The F atoms of the CFm+ ions have a small negative charge. Therefore, the attack of the C or N atoms to the negatively charged O atoms of the ≡SiOH group or ≡SiOSi≡ bridge is preferentially expected. The C/N-F distances point out that the bond strength increases with decreasing F atom number. 4.2. Chemisorption. For example, Figure 1 shows the approximate bonding geometry for the interaction between CF3+ and model b. Table 2 displays the formation enthalpies ∆FH of the LCS formed of the model molecule and one ion. All calculations show that no hypervalent Si-F, Si-C, Si-N, or O-F bonds are stable. NF4+ forms no strong bond with the model molecules by steric reasons. The same result is found for NF3+. For the other ions the O-C or O-N bonds are stable. Their bond enthalpies are in the range -59 to -92 kcal/mol. The highest stability is found for NF+ and CF2+. The other ions have smaller and nearly equal binding enthalpies. The bond formation (chemisorption) at ≡SiOSi≡ is 3-10 kcal/mol more exothermic than at ≡SiOH. The enlargement of the model by going from b to c provides no drastic changes of the enthalpies. Table 3 shows some atomic distances of the surface model molecules and their LCS with the ions. The comparison reveals a lengthening (weakening) of the C/N-F bonds as well as the
Figure 1. Approximate bonding geometry for the interaction between CF3+ and model b.
TABLE 2: Formation (∆FH), Dissociation (∆DH), and Reaction (∆RH) as Well as Activation (∆AH) Enthalpies (kcal/mol) (at 0 K) for LCS ∆FH
∆DH
∆RH
∆AH
-64.8 -70.5 -71.4
+33.2 +43.4
+22.3 +16.1 +17.1
+17.3 +20.7
-74.3 -80.2 -80.2
+35.7 +44.3
+25.0 +14.7 +12.2
+16.9 +21.0
-61.7 -66.0 -64.9
+29.8 +41.2
+9.0 -23.3 -24.2
+13.8 +13.9
-59.2 -66.9 -66.2
+31.2 +41.2
-8.8 -22.3 -25.1
+10.8 +15.4
-81.5 -91.2 -88.8
+30.6 +41.5
0.0 -21.8 -20.6
+13.2 +15.2
+
CF3 a b c CF2+ a b c CF+ a b c NF2+ a b c NF+ a b c
TABLE 3: Atomic Distances (Å) of the Model Molecules (Mdl) and Their LCS with Ions Mdl a b c CF3+ a b c CF2+ a b c CF+ a b c NF2+ a b c NF+ a b c
Sinb-Oat
Sinb-Oip
Sinb-Oop
Oat-C/N
C/N-F
1.615 1.615 1.609
1.631 1.623 1.615
1.616 1.615 1.619
1.813 1.789 1.775
1.579 1.596 1.606
1.569 1.570 1.572
1.430 1.412 1.413
1.318 1.328 1.345
1.798 1.777 1.745
1.579 1.595 1.604
1.572 1.573 1.570
1.434 1.413 1.419
1.317 1.327 1.317
1.820 1.794 1.784
1.582 1.594 1.606
1.570 1.572 1.573
1.430 1.409 1.407
1.301 1.315 1.332
1.828 1.796 1.761
1.579 1.597 1.603
1.567 1.569 1.570
1.467 1.455 1.454
1.395 1.400 1.390
1.818 1.782 1.772
1.582 1.597 1.607
1.569 1.572 1.573
1.477 1.461 1.459
1.382 1.393 1.407
Sinb-Oat bonds at the attacked O atoms and a shortening of the following Si-O bonds (Sinb-Oip and Sinb-Oop). This fact suggests that favorable starting structures for the further investigated Si-O bond breaking reactions exist. Table 4 lists the net atomic charges of the model molecules and the LCS. The formation of the LCS leads to a distribution
9822 J. Phys. Chem., Vol. 100, No. 23, 1996
Jenichen
of the positive charge of the ions over the whole structure in different extent. From the C- and N-containing ions, nearly constant portions of the positive charge are transferred to the model molecules:
C: +0.46 e (≡SiOH) +0.51 e (≡SiOSi≡) N: +0.65 e (≡SiOH) +0.70 e (≡SiOSi≡) The charge distributions of the model molecules are changed in the following way: The attacked oxygen atoms (Oat) show a different behavior. As expected, the negative Oat charge decreases by about +0.14 e for the N systems. Contrary to this, the C species cause an increase of the negative charge of the attacked oxygen by about -0.05 e. The other oxygen atoms (Oip and Oop) do not change their charge strongly. The positive charges of the Sinb (and H) atoms neighboring to Oat increase by about 0.12 e. The charges of the dangling bond hydrogen atoms at the O and Si atoms rise by about +0.06 and +0.08 e for each atom, respectively. The transferred positive charges of the ions go predominantly to the attacked O in the C systems, to the positively charged Si atoms, and partially to the H atoms at the dangling bonds. 4.3. Surface Reactions. Two reactions of the formed LCS are considered: the direct dissociation of the Si-O bond
5. Discussion
AFm +
Y O
(HO)3Si+ + YOAFm
(A)
Si O
OO
H
HH
the Si-O bond breaking over a cyclic transition state (TS) AFm +
Y
TS
Y
O Si
H
exothermic reaction (-8.8 kcal/mol). The enlargement of the model from b to c leads only to unimportant changes of the enthalpies. The very different reaction enthalpies ∆RH for reaction of type B result from the different stability of the products. Table 5 lists some atomic distances of reaction products belonging to the reactants. For the following stability considerations the reaction product of the NF3+ ion is included in the table. With decreasing number of F atoms the O-C/N bonds are strongly contracted, in particular between -OCF and -OC as well as -ONF2 and -ONF (double-bond character). This is accompanied by a lengthening of the O-Si/H bonds. Summarizing these effects, the stability of the reaction products increases with decreasing number of F atoms. In some cases the reaction enthalpies are higher than the activation enthalpies. The reason is the formation of relatively strong hydrogen bonds between the formed products after the Si-O bond break. For testing of the reliability of the method, some cases (a) are investigated using a higher level pocedure. Table 6 shows that a larger basis set (6-31G**) as well as the consideration of the electron correlation by MP2 for the geometry optimization and energy calculation leads to no drastic changes of the reaction energies. The consideration of the electron correlation by MP4 has also no important influence on the energies. For the unclear behavior of the LCS formed from ≡SiOH + NF+, the values show that the cyclic rearrangement is more endothermic than obtained by the standard procedure, so that this reaction should not be so probable as found above.
O
O
AFm –1 +
Si
F
O
OO
OO
H
HH
HH
(HO)3SiF + YOAFm–1+
(B)
Table 2 contains the dissociation enthalpies ∆DH of reaction A and the reaction ∆RH as well as activation ∆AH (≡ activation energy) enthalpies of the reaction B. The total reaction enthalpies, including the preceding chemisorptions (∆FH + ∆DH and ∆FH + ∆RH), show that reactions A and B are in all the cases exothermic. However, this is not true in every case for the individual reactions. The direct Si-O dissociation reactions A are endothermic (+30 to +45 kcal/mol). The more favorable reactions B that go over cyclic transition states require activation enthalpies between 10 and 21 kcal/ mol. The lowest barriers are caused by NF2+, NF+, and CF+ (10-16 kcal/mol). The LCS with ≡SiOSi≡ bridges have higher activation enthalpies than those with ≡SiOH groups. However, all reactions of the type B with C-containing species, except the reaction of the LCS CF+ + ≡SiOSi≡, are endothermic. The highest exothermicities (-20 to -25 kcal/ mol) are found for the reactions of CF+, NF2+, and NF+ with ≡SiOSi≡ bridges. Apart from the unclear NF+ case, NF2+ is the only ion that allow to break the ≡Si-OH bond by an
For low-temperature conditions the enthalpy values show that the surface can be blocked by CF3+ and CF2+ species. The ≡Si-OSi≡ and ≡Si-OH bonds can be broken after chemisorption of NF2+. The chemisorption of CF+ and NF+ allows to break ≡Si-OSi≡ and probably ≡Si-OH bonds. These results are in agreement with the experimental finding that NF3 discharges etch faster than CF4 ones.3,4 The products of the ≡Si-OSi≡ bond breaking reactions are ≡SiF and ≡SiOAFm-1+ groups (A ) C/N). The first is in agreement with the experimentally found SiF4. The other ones contain Si-O bonds that can be broken by the above mechanism. By reason of the strong weakening of the Si-O bonds, especially by NFn+ ions (Table 5), the Si-O break should be easy. The formed C-O and N-O bonds agree with experimentally found oxides. For the previously investigated neutral species8 a different behavior was found. As undissociated particles these molecules are not strongly bound at the surface structures. The Si-O dissociation is also possible via a cyclic transition state. The CF2 has the lowest activation enthalpies (22.0 kcal/mol for ≡SiOSi≡ and 24.0 kcal/mol for ≡Si-OH) and is, therefore, in agreement with laser experiments, the most efficient species for etching. The reactivity of the neutral species decreases in the following order:
CF2 > CF > NF > NF2 > CF3 > NF3 > CF4 The activation enthalpies for the cyclic rearrangements of the positively charged ions are lower in comparison with the neutral species. However, according to the calculated reaction enthalpies, some reactions should be thermodynamically forbidden. Different from CF2, the CF2+ is not the most efficient ion for etching since it is blocking the surface. Only the chemisorption of CF+, NF+, or NF2+, which correspond to the following
Modeling of Etching Reactions
J. Phys. Chem., Vol. 100, No. 23, 1996 9823
TABLE 4: Net Atomic Charges (e) (Mulliken Population Analysis) of the Model Molecules (Mdl) and the LCS Mdl a b c CF3+ a b c CF2+ a b c CF+ a b c NF2+ a b c NF+ a b c
Oat
Sinb
Si/H
Oip
Oop
Hip
Hop
HSi
C/N
Fip
Fop
-0.80 -0.85 -0.87
+1.47 +1.52 +1.52
+0.42 +0.93 +1.52
+0.79 -0.78 -0.78
-0.78 -0.77 -0.78
+0.42 +0.42 +0.42
+0.42 +0.42 +0.42
-0.17
-0.83 -0.92 -0.93
+1.62 +1.63 +1.62
+0.54 +0.88 +1.63
-0.78 -0.79 -0.81
-0.77 -0.77 -0.77
+0.48 +0.48 +0.47
+0.49 +0.48 +0.48
-0.10
+1.54 +1.52 +1.53
-0.33 -0.34 -0.37
-0.33 -0.34 -0.33
-0.83 -0.90 -0.91
+1.62 +1.62 +1.65
+0.54 +0.90 +1.62
-0.78 -0.79 -0.80
-0.77 -0.78 -0.78
+0.48 +0.48 +0.47
+0.49 +0.48 +0.49
-0.10
+1.20 +1.17 +1.14
-0.33 -0.34
-0.34 -0.33 -0.33
-0.82 -0.89 -0.90
+1.60 +1.61 +1.60
+0.55 +0.89 +1.61
-0.79 -0.79 -0.80
-0.77 -0.77 -0.77
+0.48 +0.48 +0.47
+0.49 +0.48 +0.48
-0.08
+0.87 +0.83 +0.83
-0.33 -0.34 -0.36
-0.63 -0.70 -0.71
+1.61 +1.62 +1.64
+0.55 -0.90 +1.62
-0.78 -0.79 -0.80
-0.77 -0.76 -0.78
+0.48 +0.48 +0.47
+0.49 +0.48 +0.48
-0.09
+0.63 +0.61 +0.59
-0.15 -0.17
-0.67 -0.74 -0.75
+1.61 +1.63 +1.61
+0.54 +0.90 +1.63
-0.79 -0.79 -0.81
-0.77 -0.77 -0.76
+0.48 +0.48 +0.46
+0.49 +0.48 +0.48
-0.09
+0.56 +0.53 +0.55
-0.18 -0.21 -0.23
TABLE 5: Atomic Distances (Å) of the Reaction Products (A ) C/N) HOAFm-1+ +
CF3 CF2+ CF+ NF3+ NF2+ NF+
H3SiOAFm-1+
(HO)3SiOAFm-1+
H-O
O-A
Si-O
O-A
Si-O
O-A
0.988 0.990 0.996 0.997 1.002 1.015
1.234 1.226 1.166 1.347 1.233 1.204
1.809 1.821 1.892 1.872 1.914 1.916
1.208 1.202 1.158 1.277 1.200 1.180
1.783 1.815 1.869 1.835 1.907 1.920
1.201 1.203 1.153 1.295 1.197 1.186
TABLE 6: Influence of Basis Set and Electron Correlation on Formation and Reaction Energies (kcal/mol) MP2/6-31G**// MP2/6-31G**// MP4/6-31G**// 3-21G(*) MP2/6-31G** MP2/6-31G** ≡SiOH + CF3+ ∆FE ∆RE ≡SiOH + CF2+ ∆FE ∆RE ≡SiOH + CF+ ∆FE ∆RE ≡SiOH + NF2+ ∆FE ∆RE ≡SiOH + NF+ ∆FE ∆RE
-67.6 +24.5 -77.6 +27.7 -64.7 +12.3 -62.6 -6.4 -84.1 +3.7
-69.0 +24.3 -79.3 +29.2 -65.8 +13.7 -63.2 -6.5 -85.9 +7.1
-63.3 +13.3
molecules in the above series, allows the break ≡Si-OSi≡ bonds by chemical reactions under low-temperature conditions. Under high-temperature conditions in plasmas, a comparison of the smaller values of Si-O dissociation enthalpies ∆RH and also ∆DH with the desorption enthalpy -∆FH shows that all the investigated ions favor the cleaving of Si-O bonds. 6. Summary Formation, reaction, and activation enthalpies of local chemisorption structures formed from CFm+ (m ) 1-3) or NFn+ (n ) 1, 2) ions and molecules modeling ≡SiOH groups and ≡SiOSi≡ bridges at SiO2 surfaces were calculated using ab initio methods. It was demonstrated that all the investigated ions form strong hypervalent C/N-O bonds to the O atoms of the local surface models. Under low-temperature conditions chemisorbed NF2+, NF+, and CF+ species allow to break chemically ≡Si-OSi≡ bonds over cyclic transition states. The ≡Si-OH bonds can be broken by chemisorption of NF2+ and
-0.15 -0.17 -0.16
probably by NF+ and CF+. On the other hand, CF3+ and CF2+ are blocking the surface. These results give an explanation that NF3 discharges lead to faster etching than by CF4 ones. However, under high-temperature conditions, as found within plasmas, all the investigated ions favor the Si-O bond breaking. The reactions of negatively charged ions (CFm- and NFn-) will be investigated in a following study. References and Notes (1) Flamm, D. L.; Donnelly, V. M. Plasma Chem. Plasma Proc. 1981, 1, 317. (2) VanRoosmalen, A. J. Vacuum 1984, 34, 429. (3) Greenberg, K. E.; Verdeyen, J. T. J. Appl. Phys. 1985, 57, 1596. (4) Ianno, N. J.; Greenberg, K. E.; Verdeyen, J. T. J. Electrochem. Soc. 1981, 128, 2174. (5) Bello, I.; Chang, W. H.; Lau, W. M. J. Vac. Sci. Technol. A 1994, 12, 1425. (6) Hikosaka, Y.; Nakamura, M.; Sugai, H. Jpn. J. Appl. Phys. 1994, 33 (P1), 2157. (7) Jenichen, A.; Johansen, H. Surf. Sci. 1988, 203, 143. (8) Jenichen, A. Surf. Sci. 1995, 331-333, 1503. (9) Tachibana, A.; Kurosaki, Y.; Kawauchi, S.; Yamabe, T. J. Phys. Chem. 1991, 95, 1716. (10) Taschibana, A.; Kawanchi, S.; Yamabe, T. J. Phys. Chem. 1991, 95, 2471. (11) Kawauchi, S.; Taschibana, A.; Yamabe, T. J. Phys. Chem. 1991, 95, 6303. (12) Jenichen, A. Int. J. Quantum Chem. 1994, 52, 117. (13) Sauer, J. Chem. ReV. 1989, 89, 199. (14) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnsen, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92; Gaussian, Inc.: Pittsburgh, PA, 1992. (15) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039. (16) Pulay, P. Modern Theoretical Chemistry; Schaefer III, H. F., Ed.; Plenum: New York, 1977; Vol. 4, Chapter 4. (17) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J. Quantum Chem., Quantum Chem. Symp. 1979, 13, 225. (18) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1989, 90, 2154; 1990, 94, 5523. (19) Pople, J. A.; Schlegel, M. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A. Int. J. Quantum Chem., Quantum Chem. Symp. 1981, 15, 269. (20) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (21) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618.
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