Russell M. Pitzer

Electronic structure of homoleptic transition metal hydrides: TiH4, VH4, CrH4, MnH4, FeH4, CoH4, and NiH4

Ab initio molecular electronic structure theory has been applied to the family of transition metal tetrahydrides TiH4 through NiH4. For the TiH4 molecule a wide range of contracted Gaussian basis sets has been tested at the self‐consistent‐field (SCF) level of theory. The largest basis, labeled M(14s 11p 6d/10s 8p 3d), H(5s 1p/3s 1p), was used for all members of the series and should yield wave functions approaching true Hartree‐Fock quality. Predicted SCF dissociation energies (relative to M+4H) and M–H bond distances are TiH4 132 kcal, 1.70 A; VH4 86 kcal, 1.64 A; CrH4 65 kcal, 1.59 A; MnH4 – 36 kcal, 1.58 A; FeH4 0 kcal, 1.58 A; CoH4 27 kcal, 1.61 A; and NiH4 18 kcal, 1.75 A. It should be noted immediately that each of these SCF dissociation energies will be increased by electron correlation effects by perhaps as much as 90 kcal. For all of these molecules except TiH4 excited states have also been studied. One of the most interesting trends seen for these excited states is the shortening of the M–H bon...

Accuracy of energy-adjusted quasirelativistic ab initio pseudopotentials: All-electron and pseudopotential benchmark calculations for Hg, HgH and their cations

The results of valence-only self-consistent field calculations on Hg n+ (n = 0, 1, 2) and HgH n+ (n = 0, 1) using nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials for Hg are compared with corresponding all-electron values from nonrelativistic (Hartree-Fock) and relativistic (Dirac-Fock) atomic as well as from nonrelativistic (Hartree-Fock) and quasirelativistic (Hartree-Fock with no-pair Hamiltonian) molecular calculations. The accuracy of the energy-adjusted ab initio pseudopotential scheme, e.g., the reproduction of the major relativistic effects, is demonstrated both for the atom and the molecule. Correlation effects are included in the quasirelativistic pseudopotential studies by means of large-scale configuration interaction calculations. The quasirelativistic pseudopotential results obtained in the intermediate coupling scheme are in excellent agreement with available experimental data.

Near Hartree‐Fock Calculations on the Ground State of the Water Molecule: Energies, Ionization Potentials, Geometry, Force Constants, and One‐Electron Properties

Near Hartree‐Fock wavefunctions have been calculated for the ground state of the water molecule using both Slater and contracted Gaussian basis sets. Total energies of −76.063 hartree were obtained with a (5s4p1d/3s1p) Slater basis and a [6s5p2d/3s1p] contracted Gaussian basis derived from an (11s7p2d/5s1p) primitive set; these energies are estimated to be within 0.003±0.002 hartree of the Hartree‐Fock limit. The Hartree‐Fock wavefunctions account for ∼70% of the dissociation energy of water. The Hartree‐Fock vertical ionization potentials (in electron volts), 11.1(2B1), 13.3(2A1), and 17.6(2B2), are too low by 1–1.5 eV as expected. With the Gaussian basis set a potential surface was computed and the equilibrium geometry and harmonic force constants were calculated. The calculated bond length, 0.941 A, and bond angle 106.6°, are in good agreement with the experimental values, 0.957 A and 104.52°. In spite of the rather good agreement for the geometry, the force constants are in error by 15%–20%. This is a...

Synthesis of High-Performance Parallel Programs for a Class of ab Initio Quantum Chemistry Models

This paper provides an overview of a program synthesis system for a class of quantum chemistry computations. These computations are expressible as a set of tensor contractions and arise in electronic structure modeling. The input to the system is a a high-level specification of the computation, from which the system can synthesize high-performance parallel code tailored to the characteristics of the target architecture. Several components of the synthesis system are described, focusing on performance optimization issues that they address.

Theoretical study of a Cu+ ion impurity in a NaF host

The Cu+ ion impurity in a NaF host has been modeled using a finite cluster of ions to represent the crystal lattice. Several approximations to the lattice potential in the region of the cluster were compared to the exact Madelung potential. The error in the calculated nearest‐neighbor distance for the pure host was found to be proportional to the error in the lattice potential. Hartree–Fock calculations were carried out for the ground 1A1g and excited 1,3Eg and 1,3T2g states of the NaF:Cu+ system. The resulting energy level structure was compared to the experimental spectra. The symmetric‐stretch potential energy curve, vibrational frequencies, and Franck–Condon factors were calculated for the 1A1g and 1,3T2g states. Using a single configuration coordinate model and a semiempirical spin–orbit coupling scheme, the relative intensities and bandwidths were calculated for absorption to the 1,3T2g states and compared to experiment.

An SCF method for hole states

An SCF method is derived for doublet states with one vacancy in an orbital within the occupied manifold (hole states). This method gives an upper bound to an excited state energy. Hence it is a stable procedure which is bounded from below and cannot collapse to a lower energy SCF state. This new procedure is compared with several other open‐shell SCF procedures which have been advocated for the ground doublet state.

The ground and excited states of C60M and C60M+ (M = O,F,K,Ca,Mn,Cs,Ba,La,Eu,U)

Restricted Hartree–Fock ab initio calculations using relativistic core potentials were performed on C60M (M=O, F, K, Ca, Mn, Cs, Ba, La, Eu, U) complexes with M as the central atom in the C60 truncated icosahedron. The icosahedral symmetry was used to great advantage in the calculations. The ground and excited states of both neutral complexes and their positive ions were studied, and the population analyses for the ground states of the complexes were obtained. The C60 cage accepts one or two electrons from electropositive elements in a formal sense, but the actual charge is usually less. Electrons in large‐radius s orbitals on the central atom tend to move outward to the carbon cage or inward to smaller‐radius d orbitals on the central atom. For the larger central atoms, ionization occurs from a cage orbital so that the ionization potentials of these complexes are almost constant.

Analytic second derivatives for Renner–Teller potential energy surfaces. Examples of the five distinct cases

Force constants at Renner–Teller stationary points fall into five distinct categories, which are readily explored using recently developed analytic energy second derivative methods. The nature of the Renner–Teller effect has been derived using symmetry principles and many‐electron perturbation theory. By considering the BH2, CH2, NH2, and CuH2 molecules, examples of all five cases are illustrated via ab initio molecular electronic structure theory.

Columbus-a program system for advanced multireference theory calculations

The COLUMBUS Program System allows high‐level quantum chemical calculations based on the multiconfiguration self‐consistent field, multireference configuration interaction with singles and doubles, and the multireference averaged quadratic coupled cluster methods. The latter method includes size‐consistency corrections at the multireference level. Nonrelativistic (NR) and spin–orbit calculations are available within multireference configuration interaction (MRCI). A prominent feature of COLUMBUS is the availability of analytic energy gradients and nonadiabatic coupling vectors for NR MRCI. This feature allows efficient optimization of stationary points and surface crossings (minima on the crossing seam). Typical applications are systematic surveys of energy surfaces in ground and excited states including bond breaking. Wave functions of practically any sophistication can be constructed limited primarily by the size of the CI expansion rather than by its complexity. A massively parallel CI step allows state‐of‐the art calculations with up to several billion configurations. Electrostatic embedding of point charges into the molecular Hamiltonian gives access to quantum mechanical/molecular mechanics calculations for all wave functions available in COLUMBUS. The analytic gradient modules allow on‐the‐fly nonadiabatic photodynamical simulations of interesting chemical and biological problems. Thus, COLUMBUS provides a wide range of highly sophisticated tools with which a large variety of interesting quantum chemical problems can be studied.

Automatic code generation for many-body electronic structure methods: the tensor contraction engine‡‡

As both electronic structure methods and the computers on which they are run become increasingly complex, the task of producing robust, reliable, high-performance implementations of methods at a rapid pace becomes increasingly daunting. In this paper we present an overview of the Tensor Contraction Engine (TCE), a unique effort to address issues of both productivity and performance through automatic code generation. The TCE is designed to take equations for many-body methods in a convenient high-level form and acts like an optimizing compiler, producing an implementation tuned to the target computer system and even to the specific chemical problem of interest. We provide examples to illustrate the TCE approach, including the ability to target different parallel programming models, and the effects of particular optimizations.