Ilya G. Kaplan

First principles study of the electronic structure and bonding of Mn2

We have examined the electronic structure and bonding of the Mn(2) molecule through multireference variational calculations coupled with augmented quadruple correlation consistent basis sets. The Mn atom has a (6)S(4s(2)3d(5)) ground state with its first excited state, (6)D(4s(1)3d(6)), located 2.145 eV higher. For all six molecular states (1)Sigma(g)(+), (3)Sigma(u)(+), (5)Sigma(g)(+), (7)Sigma(u)(+), (9)Sigma(g)(+), and (11)Sigma(u)(+)(1) correlating to Mn((6)S)+Mn((6)S), and for six undecets, i.e., (11)Pi(u), (11)Sigma(g)(+), (11)Delta(g), (11)Delta(u), (11)Sigma(u)(+)(2), and (11)Pi(g) with end fragments Mn((6)S)+Mn((6)D), complete potential energy curves have been constructed for the first time. We prove that the bonding in Mn(2) dimer is of van der Waals type. The interaction of two Mn (6)S atoms is hardly influenced by the total spin, as a result the six Sigma states, singlet ((1)Sigma(g)(+)) to undecet ((11)Sigma(u)(+)(1)), are in essence degenerate packed within an energy interval of about 70 cm(-1). Their ordering follows the spin multiplicity, the ground state being a singlet, X (1)Sigma(g)(+), with binding energy D(e) (D(0)) approximately 600 (550)cm(-1) at r(e) approximately 3.60 A. The six undecet states related to the Mn((6)S)+Mn((6)D) manifold, are chemically bound with binding energies ranging from 3 ((11)Pi(g)) to 25 ((11)Pi(u))kcal/mol and bond distances about 1 A shorter than the states of the lower manifold, Mn((6)S)+Mn((6)S). The lowest of the undecets is of Pi(u) symmetry located 30 kcal/mol above the X (1)Sigma(g)(+) state.

Nature of binding in the alkaline–earth clusters: Be3, Mg3, and Ca3

The study of the interaction energy and its many-body decomposition in a broad distance interval for the Ben, Mgn, and Can (n=2,3) clusters at the SCF and MP4 levels are performed. A comparative analysis of the obtained results allows one to conclude that the only stabilization factor in the dimers is the dispersion forces. So, the alkaline–earth dimers can be attributed to the van der Waals molecules. The trimers are stabilized by the two-body localized dispersion forces and three-body delocalized exchange forces. The binding in the alkaline–earth trimers has a mixed physical (van der Waals) and chemical (nonadditive exchange) nature. An NBO population analysis reveals a relatively large p-population in all clusters. A surprisingly large p-population at the MP4 level is also obtained for the isolated atoms.

A comparative theoretical study of stable geometries and energetic properties of small silver clusters

Abstract The systematic quantum-mechanical investigation of the stable geometries and some energetic characteristics of neutral silver clusters up to the hexamer is performed by the all-electron spin density approach with non-local corrections included. We compare these results with experimental data and with previous calculations on anionic silver clusters. We find that for the hexamer the ground-state geometry of the neutral cluster differs from the geometry of the anionic. For the neutral silver clusters Ag n the transition from a 2D conformation to a 3D occurs at n > 6. The size effects for the atomic fragmentation energy yield opposite alternations for neutral and anionic clusters.

Non-additive forces in atomic clusters: The case of Ag n

The closed recurrence formula which expresses the energy of m-body interactions through the energies of 2-, 3- and (m - 1)-body ones is obtained. The m-body contributions to the interaction energy of silver clusters are calculated by the all-electron non-local spin density method. The importance of not only 3- but also 4- and 5-body forces for cluster stability has been revealed. The stable geometry of planar and spatial silver clusters is determined by the competition of attractive additive forces and repulsive non-additive forces. The larger magnitude of non-additive forces for three-dimensional conformations in comparison with two-dimensional ones is the reason that for Ag n with n = 4-6 the most stable geometries are planar.

The Sc2 dimer revisited

Thirty two states of the homonuclear neutral diatomic Sc(2) molecule have been studied by multireference methods using basis sets of quadruple quality. For all 30 states resulting from the ground state Sc atoms, Sc((2)D(g))+Sc((2)D(g)), and two out of 80, X (5)Sigma(u) (-) and 1 (3)Sigma(u) (-), issued from the first excited channel Sc((2)D(g))+Sc(a (4)F(g)), we have constructed full potential energy curves and extracted the standard spectroscopic parameters. With the exception of X (5)Sigma(u) (-) and 1 (3)Sigma(u) (-) which are covalently bound, the 30 states related to the ground state Sc atoms are of van der Waals nature with interaction energies of 3-5 kcal/mol at distances of 7-7.5 bohr. For the X (5)Sigma(u) (-) state the proposed D(e) value is 48 kcal/mol, with respect to the adiabatic fragments and with the 1 (3)Sigma(u) (-) state just 380 cm(-1) above it.

MOLECULAR DYNAMICS STUDY OF THE AG6 CLUSTER USING AN AB INITIO MANY-BODY MODEL POTENTIAL

A general approach to construct a model potential with parameters fitted to ab initio energy surfaces, including many-body nonadditive effects, developed in our previous works is applied to the Ag6 cluster. A molecular dynamics study of structural and dynamical properties of this cluster is performed using such a potential. Two new stable two-dimensional isomers with C2v and C2h symmetries are identified as local minima of the potential surface using the simulated quenching technique. An analysis of the thermal stability as a function of the cluster temperature reveals interesting features in the meltinglike transition of Ag6. A two-step isomerization phenomenon is observed: at temperatures around 300 K, the cluster structures fluctuate among two-dimensional isomers, at higher temperatures (500 K), fast transitions occur between two- and three-dimensional cluster configurations. The simulation was extended up to the cluster fragmentation which is observed through dimer evaporation.

Nondipole bound anions: Be2− and Be3−

Electron affinities (EAs) of beryllium clusters are calculated up to the complete coupled-cluster single double triple (CCSDT) level using reasonably large basis sets with many diffuse functions. At all levels of theory, the obtained values for the adiabatic EA are large enough to be observed with standard photodetachment techniques. The vertical electron detachment energy is 0.341 eV for Be2− and is 1.470 eV for Be3− at the most precise CCSDT level. All studied beryllium anions are valence bound but the nature of binding is different in Be2− and the two Be3− isomers. The only factor of stabilization of the excess electron in Be2− is the relaxation energy. Be3−(D∞h) is stabilized by the relaxation energy and the Koopmans electrostatic and exchange energies; in Be3−(D3h), the main factors of stabilization are the correlation and relaxation energies. As was revealed in our study, in linear molecules the correlation contribution to the electron binding energy is negative, i.e., it decreases the EA.

Binding in clusters with closed-subshell atoms (alkaline-earth elements)

Abstract The study of the binding in clusters with closed subshell atoms is performed. The study is based on the accurate calculations of the Be n , Mg n , and Ca n (n =2, 3) clusters at the Moller-Plesset electron correlation level (MP4) and the SCF level, using a reasonably large basis set [6–311 + G(3df)]. The 2-and 3-body decompositions of the interaction energy at the MP4 and SCF levels, the NBO population analysis and the electron density difference maps allow to elucidate the nature of bonding in alkaline-earth clusters.

Electronic structure of ceramics at the MP2 electron correlation level

The electronic structure of the superconducting ground state of ceramics is calculated at the MP2 electron correlation level using the self-consistent embedded-cluster method. The quantum cluster is embedded in a finite array of point charges which produce the correct value of the Madelung potential at each cluster site. The results obtained reveal the strong influence of electron correlation effects on the charge distribution. The coupling between chains and planes is found to be essentially ionic while covalent bonding is predominant within both chains and planes. The symmetry of the holes on oxygens in the planes is in accordance with nuclear magnetic resonance and x-ray absorption experiments. A partially unoccupied state associated with the orbitals and is found to be present on the copper ions. The unpaired spin is located on the Cu1 atom in the chains; in the planes, the hypothetical Zhang-Rice singlet is indicated.

Role of electron correlation in nonadditive forces and ab initio model potentials for small metal clusters

The physical nature of nonadditivity in many-particle systems and the methods of calculations of many-body forces are discussed. The special attention is devoted to the electron correlation contributions to many-body forces and their role in the Be N and Li N cluster formation. The procedure is described for founding a model potential for metal clusters with parameters fitted to ab initio energetic surfaces. The proposed potential comprises two-body, three-body, and four body interation energies each one consisting of exchange and dispersion terms. Such kind of ab initio model potentials can be used in the molecular dynamics simulation studies and in the analysis of binding in small metal clusters.