A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed-shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines. © John Wiley & Sons, Inc.

General atomic and molecular electronic structure system

A single configuration model containing nonorthogonal magnetic orbitals is developed to represent the important features of the antiferromagnetic state of a transition metal dimer. A state of mixed spin symmetry and lowered space symmetry is constructed which has both conceptual and practical computational value. Either unrestricted Hartree–Fock theory or spin polarized density functional theory, e.g., Xα theory, can be used to generate the mixed spin state wave function. The most important consequence of the theory is that the Heisenberg exchange coupling constant J can be calculated simply from the energies of the mixed spin state and the highest pure spin multiplet.

Valence bond description of antiferromagnetic coupling in transition metal dimers

The computer program LOBSTER (Local Orbital Basis Suite Towards Electronic-Structure Reconstruction) enables chemical-bonding analysis based on periodic plane-wave (PAW) density-functional theory (DFT) output and is applicable to a wide range of first-principles simulations in solid-state and materials chemistry. LOBSTER incorporates analytic projection routines described previously in this very journal [J. Comput. Chem. 2013, 34, 2557] and offers improved functionality. It calculates, among others, atom-projected densities of states (pDOS), projected crystal orbital Hamilton population (pCOHP) curves, and the recently introduced bond-weighted distribution function (BWDF). The software is offered free-of-charge for non-commercial research. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jcc.24300

LOBSTER: A tool to extract chemical bonding from plane-wave based DFT

Magnetism of lanthanides in molecular materials with transition-metal ions and organic radicals.

The nature of the bonding in transition-metal compounds.

A molecular orbital analysis shows that the antiferromagnetic contributions to magnetic coupling, favoring a lowspin ground state for a dimer containing two weakly interacting metal centers, can be analyzed in terms of pairwise interactions of dimeric molecular orbitals, with the square of the splitting in energy between the members of a pair being a measure of the stabilization of the low-spin state. The effect of geometrical distortions, electronegativity, and variation of substituents on the magnetic interaction in dimeric systems is examined in detail for singly bridged L,M-X-ML, ( n = 3 , 4 , 5); Cu~C16~and other doubly bridged species where the bridging ligands are halogens, OR, pyridine N-oxides, oxalate, squarate; and the acetate bridged dimers C u ~ ( R C 0 0 ) 4 . The emphasis is on d9 Cu(I1) dimers, but other transition metal systems are also analyzed. Transition metal complexes containing more than one metal atom with unpaired electrons can generally be categorized according to their magnetic behavior into three main groups depending on the strength of the metal-metal interaction. In the noninteracting type the magnetic properties of the dimer (or polymer) a re essentially unchanged from the paramagnetic monomer. In the strongly interacting type formation of relatively strong metal-metal bonds occurs, and the molecule will display simple diamagnetic behavior (for even numbers of electrons). In this paper the properties of weakly interacting metal ions will be investigated. In such compounds this weak coupling between the electrons of the two metal ions leads to low-lying excited states of different spin which can be populated a t thermal energies (SI000 cm-I). The resulting magnetic behavior will be antiferromagnetic or ferromagnetic, depending on whether the low spin (spins paired) or high spin (spins parallel) state is the ground state, respectively. These interactions-often termed superexchange because of the large distances involved (3-5 A) between the metal ions-have been observed in a wide variety of compounds. I 5 In experimental studies the magnetic interaction between spins SA and Sg for atoms A and B is usually written in a form suggested originally by Heisenberg, Dirac, and Van

Orbital interactions in metal dimer complexes

In both extended Hiickel and limited basis set ab initio SCF molecular orbital calculations an intriguing phenomenon occurs, especially in calculations on transition metal complexes. Some of the lower orbitals, which are made up primarily of ligand 2s combinations, mix into themselves the metal virtual orbitals, 4s and 4p, in an out-of-phase manner, opposite to what one would have predicted from a simple perturbation analysis. We call this counterintuitive orbital mixing (COM). The conse- quences of COM are lowered metal to ligand overlap populations and negative Mulliken gross atomic populations of the virtual orbitals. We analyze the prerequisites for occurrence of COM in both semiempirical and SCF methods and propose a modified Wolfsberg-Helmholz formula for reducing the magnitude of the effect in EH calculations.

Counterintuitive Orbital Mixing in Semiempirical and ab Initio Molecular Orbital Calculations

Rotational barriers and conformational preferences are a primary probe of bonding in olefin complexes. Such bar- riers in ethylene-ML2-s are analyzed in terms of differential interactions between the frontier orbitals of the ML, fragment and the ethylene 7r and x*. It is found that the large barrier to internal rotation about the M-ethylene axis in ethylene-ML2 complexes, favoring the in-plane orientation, is due to loss of overlap between ethylene 7r* and an ML2 b2 orbital-the domi- nant interaction in these compounds. An analogous situation exists for rigid rotation in ethylene-ML4 within the trigonal-bipy- ramidal geometry. A much lower energy pathway for this complex is found if rotation is accompanied by pseudorotation. The barrier in square-planar ethylene-ML3 compounds of the Zeise's salt type, on the other hand, is largely set by steric factors which favor the upright geometry. Various strategies are devised to lower the barrier or reverse the conformational preference in these complexes. This may be accomplished by changing the electronic or steric properties of the ligands on the metal or the ethylene. Finally unsymmetrically substituted olefin complexes are examined. In the ML3 case the metal-carbon bond to the carbon bearing the weaker donor or weaker acceptor should be the stronger or shorter one. In the ML2 and ML4 complexes of ethylene the acceptor effect is accentuated, that of the donor less important.

Jack C. Thibeault

Papers

Orbital interactions in metal dimer complexes

Counterintuitive Orbital Mixing in Semiempirical and ab Initio Molecular Orbital Calculations

Ethylene complexes. Bonding, rotational barriers, and conformational preferences