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Electronic, Magnetic, and Transport Properties of Fe-COT Clusters: A Theoretical Study Jing Huang,† Qunxiang Li,*,‡ Ke Xu,‡ Haibin Su,§ and Jinlong Yang‡ School of Materials and Chemical Engineering, Anhui UniVersity of Architecture, Hefei, Anhui 230022, People’s Republic of China, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China, and DiVision of Materials Science, Nanyang Technological UniVersity, 50 Nanyang AVenue, 639798, Singapore ReceiVed: February 21, 2010; ReVised Manuscript ReceiVed: May 16, 2010
Using density functional theory calculations combined with nonequilibrium Green’s function method, we report the electronic, magnetic, and transport properties of iron-cyclooctatetraene (Fe-COT) sandwich clusters. Four Fen COTn+1 (n ) 1-4) clusters with linear sandwich structure are highly stable because of the strong Fe-COT coupling. The ground state of Fe-COT clusters is ferromagnetic, Fe atoms couple ferromagnetically to the neighboring COT rings, and the large total magnetic moments increase with the number of Fe atoms. The spin-polarized transport calculations indicate that Fe-COT clusters coupled to gold electrodes act as nearly perfect spin-filters. The revealed properties indicate that the Fe-COT clusters would be ideal materials for promising molecular spintronics. I. Introduction Nowadays, molecular spintronics has been attracting enormous attention because it holds promise for future applications in high-density information storage and quantum computing. As promising building units, 1D organometallic wires have been extensively studied since the discovery of ferrocene,1 especially sandwich clusters due to the unique electronic, magnetic, and transport properties. Previous studies focused on the first transition metal-benzene (TM-Bz) and TM-cyclopentadienyl (TM-Cp) clusters.2-11 For example, Fe-Cp and Mn-Bz wires were predicted to be half-metallic based on the first-principles calculations.4 Ab initio electron transport calculations have shown that FenCpn+1 (n ) 1-4) clusters connected to Fe electrodes5 and V-Bz clusters sandwiched between ferromagnetic electrodes or graphene electrodes can act as nearly perfect molecular spin filters.8,9 Note that the transmission spin polarization depends nontrivially on the interface structures between the electrodes and the molecule. For example, these asymmetric sandwich molecular wires (SMWs) based on FenCpn+1 (n ) 2,3) have a nearly perfect spin filter efficiency (SFE), whereas the SFE through FenCpn clusters is poor.8 More recent attention has been paid to organometallic based on 1,3,5,7-cyclooctatetraene (COT) since the stable uranocene (U(COT)2) was successfully synthesized.12 Many lanthanide complexes (Ln ) Ce, Nd, Eu, Tb, Ho, Tm, and Yb) and transition-metal complexes (TM ) V, Fe, Ni, and Ag) sandwiched between COT rings have been successfully produced by using laser vaporization and molecular beam methods in the past few years.13-20 Using molecular beam magnetic deflection technique, Nakajima et al.20 investigated the electronic and magnetic properties of Eu-COT clusters and found that Eu-COT has large magnetic moment (MM) and the total MM linearly increases with the number of Eu atoms. Jaeger et al.16 and Kandalam et al.21 performed photodissociation and anion photoelectron spectroscopy studies on Fe-COT clusters, respec* Corresponding author. E-mail:
[email protected]. † Anhui University of Architecture. ‡ University of Science and Technology of China. § Nanyang Technological University.
tively. For Ln-COT clusters, these experimental and theoretical investigations mainly focus on the stability, charge transfer, electronic structure, and magnetic properties;22 only few attempt to explore the electron transport property of Ln-COT clusters.23 Moreover, there appears to be a lack of theoretical study on transport behavior through TM-COT clusters. In this article, we present first-principles calculations combined with nonequilibrium Green’s function (NEGF) technique to explore electronic, magnetic, and transport properties of Fe-COT clusters [FenCOTn+1 (n ) 1-4)]. The calculated results indicate that the ground state of Fe-COT clusters is ferromagnetic and the total magnetic moments increase with the number of Fe atoms. Moreover, Fe-COT clusters connected to gold electrodes can act as nearly perfect spin filters. Our prediction implies that Fe-COT clusters would be ideal materials for molecular spintronics. II. Computational Details Structure optimization, density of states, and band structures calculations of four FenCOTn+1 (n ) 1-4) clusters (Figure 1) are performed using the Spanish initiative for electronic simulations with thousands of atoms (SIESTA) code24 based on spin-polarized density functional theory. To calculate electron transport properties through Fe-COT clusters, they are sandwiched between two Au(100) surfaces, as shown in the top panels of Figure 2a-d. We carried out the transport calculations by using the fully self-consistent NEGF combined with firstprinciples calculations, implemented in ATK package,25,26 which has been successfully used to explain many experimental results.27 The two-probe system can be divided into three parts including the scattering region (extended molecule) and left and right electrodes. The scattering region consists of a Fe-COT cluster and two(three) surface layers of the left(right) electrodes, where all screening effects are included in the contact region and charge distribution in left and right electrodes is the same as that of the bulk phase. In our calculations, we employ Troullier-Martins nonlocal pseudopotential and linear combinations of atomic orbitals as basis set. Double-ζ plus polarization (DZP) basis sets are used for non-Au atoms, and only single-ζ
10.1021/jp101554c 2010 American Chemical Society Published on Web 06/17/2010
Properties of Fe-COT Clusters: A Theoretical Study
J. Phys. Chem. C, Vol. 114, No. 27, 2010 11947 The transmission spectrum is calculated by
T(E, V) ) Tr[ΓL(E, V)G(E, V)ΓR(E, V)G+(E, V)]
(1)
where ΓL(R) stands for the coupling matrix between the left(right) electrode and scattering region and G(E,V) is the retarded Green’s function of scattering region. The current through the molecular junction is obtained by
I(V) )
2e h
∫ T(E, V)[f(E - µL) - f(E - µR)] dE (2)
Here f(E - µL(R)) and µL(R) is the Fermi function and chemical potential of left (right) electrode, respectively. Figure 1. Optimized structures of FenCOTn+1 (n ) 1-4) clusters. The Fe-C and vertical Fe-COT distances are shown in the Figures. The positions of Fe atoms and COT rings in Fe-COT clusters are defined with labels Fe1/2 and COT1/2/3.
Figure 2. Spin-resolved transmission spectra of Fe-COT clusters sandwiched between two Au(100) surfaces. (a) FeCOT2, (b) Fe2COT3, (c) Fe3COT4, and (d) Fe4COT5. Here insets at the top panels of parts a-d stand for the examined Fe-COT molecular junctions, where vertical blue lines represent borders of the central scattering region. The red and black lines stand for transmission spectra of the minority and majority spin states, respectively. Empty triangles are molecular projected self-consistent Hamiltonian eigenvalues.
plus polarization (SZP) basis sets are employed for Au atoms to save the computational effort. The grid integration is defined by an energy cutoff of 150 Ry, whereas the exchange and correlation functional is treated at the level of Ceperley-Alder local-density approximation.28 To neglect the interaction between the neighboring fragments, these isolated Fe-COT clusters are placed in an appropriate supercell (20 × 20 × 30 Å3) with periodic boundary conditions when we carry out geometric optimizations and electronic structure calculations. Test calculations with larger basis set and cutoff energy yield the very similar results.
III. Results and Discussion First, we test the reliability of these adopting computational parameters for Fe-COT clusters by calculating the adiabatic electron affinity (EA) value of half-sandwich Fe-COT cluster: one Fe atom locates above the center of a COT ring. The calculated EA is ∼0.95 eV, which is close to the experimental measurement (1.18 eV) and previous theoretical result (1.04 eV).21 Before building the Fe-COT junctions, we optimize the geometric structures of Fe-COT clusters.29 The stable structures of FenCOTn+1 (n ) 1-4) clusters are shown in Figure 1a-d. The interatomic C-H distances in different COT rings are almost the same (∼1.10 Å), and the C-C bond length varies slightly from 1.39 to 1.42 Å. The vertical distance between two neighboring COT rings in FeCOT2 and Fe2COT3 clusters is ∼3.20 Å, and the average Fe-C distance is ∼2.45 Å, whereas the neighboring COT-COT distances are ∼3.35 Å, and the average Fe-C distance is ∼2.50 Å in Fe3COT4 and Fe4COT5 clusters. Note that the Fe atom does not locate exactly at the middle of two neighboring COT rings. For example, for the Fe2COT3 cluster, the vertical distance between the Fe atom and the terminal COT ring (1.54 Å) is shorter than the distance (1.67 Å) between the Fe atom and the central COT ring. It should be pointed out that the Fe atoms prefer the terminated COT rings of three investigated FenCOTn+1 (n ) 2-4) clusters. This observation is related to the asymmetric charge density distribution at the outside and inside of the edge COT rings in these Fe-COT clusters, which results in the Fe atoms slightly moving to the edge COT rings. To examine their stability, we define the average binding energies (BEs) as: BE(n, n + 1) ) (1/n)[nEFe +(n + 1)ECOT) - EFenCOTn+1], where EFenCOTn+1, ECOT, and EFe are the total energies of the optimized Fe-COT clusters, COT rings, and Fe atoms, respectively. The corresponding calculated BE for FenCOTn+1 for n ) 1-4 is 5.50, 5.46, 5.40, and 5.40 eV, respectively. It is clear that the Fe-COT cluster can exist stably; the bonding between Fe atoms and COT rings are not sensitive to the cluster size, and these binding energies are less than that of Eu-COT clusters.22 Before sandwiching Fe-COT clusters between two Au(100) surfaces, we first determine the energetic favorable adsorption geometry of four Fe-COT clusters on Au(100) surface. Three different adsorption sites including hollow, bridge, and atop sites are examined. The calculated adsorption energy of Fe2COT3 cluster on Au(100) at hollow site is lower by about 0.37 and 1.37 eV than that of atop and bridge sites, respectively, which indicates that Fe-COT clusters prefer to the hollow site of Au(100) surface. In the investigated molecular junctions, the optimized Fe-COT clusters are connected to the hollow sites
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of two Au(100) surfaces, and the electrodes are modeled by 4 × 4 supercells with periodic boundary conditions. Then, we optimize the anchoring distances between the terminated COT rings of Fe-COT clusters and Au(100) surfaces (dAu-COT) for these examined junctions. The equilibrium dAu-COT is predicted to be ∼2.36 Å, varying slightly within 0.04 Å. Then, all atomic positions of Fe-COT clusters are relaxed. We find that the interatomic C-H and C-C distances in molecular junctions are very close to that of isolated Fe-COT clusters; the corresponding bond length is about 1.11 and 1.42 Å, respectively. Now we turn to investigate the spin-polarized transport properties through these Fe-COT clusters. Figure 2 shows the spinpolarized energy dependence zero-bias transmission spectra within energy window from -2.0 to 2.0 eV. Here the Fermi level is set as zero for clarity, and the upside-down triangles stand for the eigenvalues of the molecular projected selfconsistent Hamiltonian,26 which can be referred as perturbed molecular orbitals (MOs) because of the presence of gold electrodes. Clearly, there are several obvious features: (1) The energy positions of the perturbed MOs relative to the Fermi level match nicely with the transmission peaks for both spin channels. (2) For FenCOTn+1 (n ) 1-4) clusters, the calculated energy gap between the perturbed highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for spin-down state (minority spin channel) is 0.96, 0.64, 0.47, and 0.34 eV, which are less than the energy gap for the spin-up state (majority spin channel), corresponding to 2.96, 2.52, 2.59, and 2.45 eV, respectively. (3) Excepting FeCOT2 cluster, the obvious difference of the transmission spectra of two spin channels is observed for FenCOTn+1 (n ) 2-4) systems, and the minority electron through Fe-COT cluster has several broad transmission peaks locating from -1.0 and 1.0 eV, whereas there is no significant transmission peak within the range from -0.5 to 1.0 eV for the majority electron. To quantify the transmission spin polarization, the SFE is defined as SFE ) |Tmin(EF) - Tmaj(EF)|/Tmin(EF) in our calculations, where Tmin and Tmaj stand for the transmission spectra coefficient of the minority and majority spin channels at the Fermi level, respectively. The SFE is predicted to be about 65.2, 98.1, 99.8, and 99.9% for FeCOT2, Fe2COT3, Fe3COT4, and Fe4COT5, respectively. The SFE increases with the length of the Fe-COT clusters, and SFE of Fe-COT with n ) 2, 3, and 4 is close to 100%. This observation is easy to understand according to the calculated transmission spectra and the perturbed MOs. As shown in Figure 2, the transport behavior through Fe-COT clusters under the small bias voltage is mainly determined by the transmission peak originated from the perturbed HOMO of the minority spin channel. With the increase in molecular length, both the perturbed HOMO and LUMO shift toward the Fermi level, which narrows the HOMO-LUMO gap and results in the increase in SFE. These results suggest that the FenCOTn+1 (n ) 2-4) clusters can be used to design nearly perfect spin filter. To verify this prediction, as an example, the spin-polarized current-voltage (I-V) curves through Fe2COT3 cluster in the bias voltage range of 0.0 to 1.2 V are calculated and shown in Figure 3. At each bias voltage, the current is determined selfconsistently under the nonequilibrium condition using the Landauer-Bu¨tiker formula. The current of minority spin channel through the Fe2COT3 cluster is larger than that of majority spin channel, and the I-V curve displays obvious spin-filter behavior. For the majority channel, the I-V curve has flat characteristics and the current increases slowly with the bias voltage. However, the current of minority spin channel through the Fe2COT3 cluster
Huang et al.
Figure 3. Spin-polarized I-V curves for Fe2COT3 cluster sandwiched between gold electrodes. The black and red lines stand for minority and majority spin channels, respectively.
TABLE 1: Energy Difference (∆E, in millielectronvolts) between Different Magnetic Configurations (MCs), MCs, Total Magnetic Moment (µT) and the Average Localized Magnetic Moment on Fe Atoms (µFe) and COT Rings (µCOT in µB), and Spin Filter Efficiency (SFE in percent) of Fe-COT Clustersa cluster FeCOT2 Fe2COT3 Fe3COT4 Fe4COT5
∆E
MC
µT
µFe
µCOT1
µCOT2
0 23 0 4 37 0 30 18 19 32 14
vv vV vvv vvV vVv vvvv vVvV vvVV vvvV vVvv vVVv
6.0 10.0 0.0 12.5 4.0 4.0 16.5 0.0 0.0 0.0 8.0 8.0
3.39 3.40 3.42 3.42 3.40 3.41 3.42 3.39 3.40 3.40 3.40 3.40
1.31 1.11
0.98
65.2 98.1
0.50
0.62
99.8
0.49
0.61
µCOT3
0.62
SFE
99.9
a Here COT1-3 specifies the COT rings from the termination to the center in the clusters.
increases almost linearly with bias voltage. The current difference between majority and minority spin channels under bias voltage can be quantified by the ratio of current defined as R(V) ) |Imin(V)/Imaj(V)|. This calculated R(V) is up to 30 (varying from 20 to 30 in the bias voltage window from 0.0 to 1.2 V). Actually, the observed significant SFE of Fe-COT clusters is insensitive to the contact configuration. As an example of Fe-COT asymmetric junctions, we perform the calculations of the spinpolarized transmission spectra of Fe3COT3 junction. The SFE is predicted to be ∼95.1%. These theoretical results imply that the Fe-COT cluster can be an ideal promising building block for designing molecular spintronics devices. To understand these observed obvious SFE, we turn to explore the electronic structures and magnetic properties of Fe-COT clusters. Here, to examine the stability of spin arrangement of Fe-COT clusters, we take all possible different magnetic configurations (MCs) into consideration. The main calculated results, including the energy differences (∆E, in millielectronvolts) between two different MCs, total magnetic moments (µT), and localized magnetic moments on Fe atoms (µFe) and COT rings (µCOT in µB), are summarized in Table 1. The spatial distribution of spin-density for four Fe-COT clusters is shown in Figure 4. The ∆E between the ferromagnetic and ferrimagnetic/antiferromagnetic states varies from 4 to 37 meV, which depends on Fe-COT cluster size and spin arrangement. The
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Figure 4. Spin density for Fe-COT clusters. (a) FeCOT2, (b) Fe2COT3, (c) Fe3COT4, and (d) Fe4COT5. The isovalue for the red isosurfaces is all set to be 0.005 e/Å3.
relative small energy difference was also predicted for europiumcyclootetatrene and TMn(ferrocene)n+1 (TM ) Sc, Ti, V, Mn) clusters,7,22,23 which means that the Fe-COT clusters can be switched between ferromagnetic and ferrimagnetic/antiferroMCs by the thermal fluctuations. It is expected that the obvious spin filter effect through Fe-COT clusters could be only observed at low temperature in experiments. Although the predicted ∆E value is very small, it is clear that the ground state of four FeCOT clusters is ferromagnetic. Fe atoms energetically prefer to couple ferromagnetically in four examined Fe-COT clusters. The spin-density maps indicate quite localized magnetic moments at Fe atoms, and the COT rings exhibit small magnetic moments. Note that Fe atoms couple ferromagnetically to COT rings, which is in contrast with Eu-COT clusters22,23 and TMBz wires3 in which the metal atom antiferromagnetically couples to COT or Bz ring. Here we also perform additional calculations on the electronic structures of 1D Fe-COT wire. The obtained results indicate that the ground state for Fe-COT wire is antiferromagnetic and the Fe atom antiferromagnetically couples to the neighboring COT rings. To gain further insight into the magnetic properties of these Fe-COT clusters, we calculate the spin-polarized partial density of states (PDOSs) of Fe atoms and COT rings, as shown in Figure 5. It is clear that all PDOSs for spin-down and spin-up states are obviously asymmetric. The large magnetic moments of Fe-COT clusters are revealed from the fact that the spindown states were filled more than the spin-up states for FeCOT clusters. The 3d orbitals of Fe atoms in Fe-COT clusters are obviously spin-split in the ligand environment of COT rings. The average µFe is ∼3.4 µB in all Fe-COT clusters, whereas the µCOT varies from 0.49 to 1.31 µB, depending on its position in the cluster and the Fe-COT cluster size. Note that the average µFe in Fe-COT clusters is significantly larger than that of transition-metal atom in the TM-Cp and TM-Bz wires (the average µFe in Fe-Bz wire is ∼1.0 µB),4 but they are less than that of Eu-COT clusters (the average µEu in Eu-COT clusters is ∼7.0 µB).22,23 In FeCOT2 and Fe2COT3 clusters, the total MM (µT) is about 6.0 and 10.0 µB, respectively. The neighboring COT-COT distance is ∼3.20 Å, and each COT ring has about one unpaired electron coupling ferromagnetically to Fe atom(s), whereas the Fe-COT ferromagnetic interaction becomes somewhat weak because of the relatively long COT-COT distances (3.33 to 3.36 Å, as shown in Figure 1) in Fe3COT4 and Fe4COT5 clusters. As a result, µCOT in Fe3COT4 and Fe4COT5 clusters varies from 0.49 to 0.62 µB, and the corresponding µT is 12.5 and 16.5 µB, respectively. These electronic structure results imply that the obvious spin-filter current-voltage characteristic through Fe-COT clusters originates from the significant differ-
Figure 5. Spin-resolved partial density of states of Fe atoms and COT rings in Fe-COT clusters. (a) Fe2COT3 and (b) Fe3COT4. Here the labels COT1/2/3 and Fe1/2 are defined in Figure 1 for clarity.
ence between two spin channels, the ferromagnetically Fe-COT coupling, and the large total magnetic moment. IV. Summary In summary, the electronic structures and magnetic and transport properties of four Fe-COT clusters are investigated by spin-resolved DFT and NEGF techniques. The ground state of all Fe-COT clusters is ferromagnetic, and Fe atoms couple ferromagnetically to COT rings. The large total magnetic moment of Fe-COT clusters increases with the number of Fe atoms. The transport calculations show that the Fe-COT clusters coupled between Au(100) surfaces can act as nearly perfect spin filters. These revealed results indicate that the Fe-COT clusters could be used to build molecular spintronics device. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (nos. 20903003, 20773112, and 50721091), by National Key Basic Research Program (no. 2006CB922004), by Knowledge Innovation Program of the Chinese Academy of Sciences (no. KJCX2YW-W22), by the Scientific Research Foundation of Anhui University of Architecture (no. K0241801), by the USTC-HP HPC project, and by the SCCAS and Shanghai Supercomputer Center. Work at NTU is partially supported by A*STAR SERC grant (no. 0521170032). References and Notes (1) Kealy, T. J.; Pauson, P. L. Nature 1951, 168, 1039. (2) Wang, J.; Acioli, P. H.; Jellinek, J. J. Am. Chem. Soc. 2005, 127, 2812–2813.
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(3) Xiang, H. J.; Yang, J. L.; Hou, J. G.; Zhu, Q. S. J. Am. Chem. Soc. 2006, 128, 2310–2314. (4) Shen, L.; Yang, S. W.; Ng, M. F.; Ligatchev, V.; Zhou, L.; Feng, Y. P. J. Am. Chem. Soc. 2008, 130, 13956–13960. (5) Zhou, L. P.; Yang, S. W.; Ng, M. F.; Sullivan, M. B.; Tan, V. B. C.; Shen, L. J. Am. Chem. Soc. 2008, 130, 4023–4027. (6) Wang, L.; Cai, Z.; Wang, J.; Lu, J.; Luo, G.; Lai, L.; Zhou, J.; Qin, R.; Gao, Z.; Yu, D.; Li, G.; Mei, W. N.; Sanvito, S. Nano Lett. 2008, 8, 3640–3644. (7) Zhang, X.; Wang, J.; Gao, Y.; Zeng, X. C. ACS Nano 2009, 3, 537–545. (8) Maslyuk, V. V.; Bagrets, A.; Meded, V.; Arnold, A.; Evers, F.; Brandbyge, M.; Bredow, T.; Mertig, I. Phys. ReV. Lett. 2006, 97, 097201. (9) Koleini, M.; Paulsson, M.; Brandbyge, M. Phys. ReV. Lett. 2007, 98, 197202. (10) Goto, A.; Yabushita, S. Chem. Phys. Lett. 2008, 454, 382–386. (11) Wu, F.; Tjornhammar, R.; Kan, E. J.; Li, Z. Y. Front. Phys. China 2009, 4, 403–407. (12) Streitwieser, A.; Muller-Westerhoff, U. J. Am. Chem. Soc. 1968, 90, 7364–7366. (13) Kurikawa, T.; Negishi, Y.; Hayakawa, F.; Nagao, S.; Miyajima, K.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 1998, 120, 11766–11772. (14) Li, J.; Bursten, B. E. J. Am. Chem. Soc. 1998, 120, 11456–11466. (15) Miyajima, K.; Kurikawa, T.; Hashimoto, M.; Nakajima, A.; Kaya, K. Chem. Phys. Lett. 1999, 306, 256–262. (16) Jaeger, T. D.; Duncan, M. A. J. Phys. Chem. A 2004, 108, 11296– 11301. (17) Takegami, R.; Hosoya, N.; Suzumura, J.; Nakajima, A.; Yabushita, S. J. Phys. Chem. A 2005, 109, 2476–2486.
Huang et al. (18) Takegami, R.; Hosoya, N.; Suzumura, J.; Yada, K.; Nakajima, A.; Yabushita, S. Chem. Phys. Lett. 2005, 403, 169–174. (19) Hosoya, N.; Takegami, R.; Suzumura, J.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, A. J. Phys. Chem. A 2005, 109, 9–12. (20) Miyajima, K.; Knickelbein, M. B.; Nakajima, A. J. Phys. Chem. A 2008, 112, 366–375. (21) Li, X.; Eustis, S. N.; Bowen, K. H.; Kandalam, A. J. Chem. Phys. 2008, 129, 124312. (22) Zhang, X.; Ng, M. F.; Wang, Y.; Wang, J.; Yang, S. W. ACS Nano 2009, 3, 2515–2522. (23) Xu, K.; Huang, J.; Lei, S. L.; Su, H. B.; Boey, Freddy, Y. C.; Li, Q. X.; Yang, J. L. J. Chem. Phys. 2009, 131, 104704. (24) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejo´n, P.; Portal, D. S. J. Phys.: Condens. Matter 2002, 14, 2745–2779. (25) Brandbyge, M.; Mozos, J. L.; Ordejo´n, P.; Taylor, J.; Stokbro, K. Phys. ReV. B 2002, 65, 165401. (26) Taylor, J.; Guo, H.; Wang, J. Phys. ReV. B 2001, 63, 245407. (27) (a) Huang, J.; Li, Q. X.; Ren, H.; Su, H. B.; Shi, Q. W.; Yang, J. L. J. Chem. Phys. 2007, 127, 094705. (b) Huang, J.; Li, Q. X.; Su, H. B.; Yang, J. L. Chem. Phys. Lett. 2009, 479, 120–124. (c) Li, Z.; Kosov, D. S. J. Phys. Chem. B 2006, 110, 19116–19120. (28) Ceperley, D. M.; Alder, B. J. Phys. ReV. Lett. 1980, 45, 566. (29) We examine two possible eclipsed (with D8h symmetry) and staggered (D8d) conformations of two neighboring COT rings in four FeCOT clusters and find that the eclipsed structures are energetically stable. (The energy difference of two different conformations is very small within 10 meV.)
JP101554C
