A multireference second-order perturbation theory (MRPT2) has been developed which allows the use of reference wave functions with large active spaces and arbitrary configuration selection. Internally contracted configurations are used as a basis for all configuration subspaces of the first-order wave function for which the overlap matrix depends only on the second-order density matrix of the reference function. Some other subspaces which would require the third- or fourth-order density matrices are left uncontracted. This removes bottlenecks of the complete active space second order pertubation theory (CASPT2) caused by the need to construct and diagonalize large overlap matrices. Preliminary applications of the new method for 1,2-dihydronaphthalene (DHN) and free base porphin are presented in which the effect of applying occupancy restrictions in the reference wave function (restricted active space second-order perturbation theory, RASPT2) and reference configuration selection (general MRPT2) on electro...

Multireference perturbation theory for large restricted and selected active space reference wave functions

The photoelectron spectra of some 40 transition metal compounds have been measured using AlKα(1487 eV) and MgKα(1254 eV) x‐rays. The compounds included both simple salts (halides and chalcogenides) and hexacoordinated complexes (cyano‐ and fluoro‐) of Cr(III), Mn(II, III), Fe(II, III), and Co(III). From these data we have determined the chemical shifts of the core electrons and related these results to a calculated charge based on Pauling's electronegativities. In addition, multiplet splitting has been obtained for photoionization in the 3s shell and is discussed in terms of the exchange interaction between the partially filled 3s and 3d orbitals. To help in this explanation, calculations were made using both (1) Hartree‐Fock solutions of the wavefunctions for free ions and (2) a qualitative evaluation of the behavior of the exchange integral. The value of using x‐ray photoelectron spectroscopy for studying chemical bonding for transition metal compounds is amply illustrated.

Use of X‐Ray Photoelectron Spectroscopy to Study Bonding in Cr, Mn, Fe, and Co Compounds

http://people.bu.edu/parkhs/Papers/liuCMAME2004.pdf

An introduction to computational nanomechanics and materials

We describe an ab initio procedure for extracting the Pauli exchange antisymmetry (“steric”) contributions to molecular potential energy in the framework of self-consistent-field molecular orbital (SCFMO) theory. This “natural steric analysis” method is based on natural bond orbital (NBO) representation of the SCFMO wave function, which allows the steric exchange energy to be approximated as an energy difference between “preorthogonal” and final NBOs, analogous to the procedure of Sovers et al. [J. Chem. Phys. 49, 2592 (1965)]. We show how the total NBO steric exchange energy can in turn be approximated in terms of pairwise-additive interactions between localized bonding units, comparable to the empirical steric potentials of molecular mechanics models. The accuracy of NBO steric analysis is tested in applications to various rare-gas interactions (expected to be of pure steric exchange type), and excellent agreement is found with the full ab initio potential curves over a wide range of separations. Limitations of the simple pairwise-additive model of steric exchange interactions are noted and characterized.

Natural bond orbital analysis of steric interactions

Abstract The glossary contains definitions and explanatory notes for more than 450 terms used in the context of multidisciplinary research and publications related to applications of modern theoretical concepts, computational and graph-theoretical methods to investigation into structure, reactivity, spectroscopic and other physical and physicochemical properties of organic, organometallic and metal coordination compounds. The aim of the glossary is to provide guidance on terminology used in theoretical organic chemistry and to contribute to the elimination of inconsistencies and ambiguities in the meanings of terms in the area.

Glossary of terms used in theoretical organic chemistry

A composite approach to quantum chemistry, combining conceptual material with formal maths Topics discussed include the Schrodinger wave equation, the quantum theory of angular momentum, molecular structure and the electronic structure of linear and non-linear molecules

Frank L. Pilar

Papers

Elementary quantum chemistry