J.H. van Lenthe

The GAMESS-UK electronic structure package: algorithms, developments and applications

A description of the ab initio quantum chemistry package GAMESS-UK is presented. The package offers a wide range of quantum mechanical wavefunctions, capable of treating systems ranging from closed-shell molecules through to the species involved in complex reaction mechanisms. The availability of a wide variety of correlation methods provides the necessary functionality to tackle a number of chemically important tasks, ranging from geometry optimization and transition-state location to the treatment of solvation effects and the prediction of excited state spectra. With the availability of relativistic ECPs and the development of ZORA, such calculations may be performed on the entire Periodic Table, including the lanthanides. Emphasis is given to the DFT module, which has been extensively developed in recent years, and a number of other, novel features of the program. The parallelization strategy used in the program is outlined, and detailed speedup results are given. Applications of the code in the areas of enzyme and zeolite catalysis and in spectroscopy are described.

The direct CI method

A thorough analysis of the direct CI method as applied to the case of a general set of reference configurations coupled to all single and double substitutions is presented. It is pointed out that there is no single strategy which proves optimal under all circumstances. A variety of procedures are therefore presented together with rules to enable the selection of the most favourable under a given circumstance. Much emphasis has been placed on organizing the calculations via a series of matrix multiplications, which enables a vector or array processing computer to be used to best effect. Some consideration is given to using an atomic integral (rather than molecular integral) driven scheme for some interactions, thus removing the necessity for a complete transformation of the molecular integrals to a molecular orbital basis, and the advantages and disadvantages of so doing are discussed. Improved procedures for carrying out both full and partial transformations of the molecular integrals are described. A num...

THE BASIS SET SUPERPOSITION ERROR IN CORRELATED ELECTRONIC STRUCTURE CALCULATIONS

Abstract A thorough investigation of the proper scheme to correct for basis set superposition errors is performed for the He dimer within the CEPA(1) method. The BSSE proves, even for this small system, extremely difficult to avoid, and it is in no way negligible. Through comparison with perturbation-theory estimates of the interaction energy, we conclude that the full counterpoise correction should be applied rather than a scheme omitting the occupied orbitais of the ghost.

The ZORA formalism applied to the Dirac-Fock equation.

Abstract The zeroth-order regular approximation (ZORA), a two component approximation to the Dirac equation that was earlier formulated and tested within the framework of density functional theory, is generalized to a treatment based on the Dirac-Fock equation. The performance of the ZORA equation and an improvement known as scaled ZORA is investigated, in particular with respect to orbital energies and various radial expectation values in the case of the xenon and radon atoms. The results of the simple ZORA approximation are shown to be quite close to the full Dirac-Fock method, except in the deep core region where the scaled version of the method is needed. It is found that a further approximation in which the density is calculated from the two-component ZORA orbitals alone gives satisfactory results, which is an important result from a practical point of view since in this way one can avoid calculating any two-electron integrals involving small-component basis functions.

Convergence to the configuration‐set limit in multireference configuration‐interaction calculations on the He dimer

The multireference configuration‐interaction (MR‐CI) method is used to calculate the binding energy of the He dimer. The convergence of the binding energy to the configuration‐set limit (full‐CI) is followed by progressively extending the multireference configuration set. Two variants of the Pople size‐extensivity correction are applied. The distance dependence of the corrections and hence the effect upon the binding energy turns out to be very small. The effect of orbital optimization is studied and it is shown that it is sufficient to optimize the orbitals used for the multireference space in an atomic multiconfiguration self‐consistent field (MCSCF) calculation. In a basis of 50 atomic orbitals, the full‐CI binding energy of −9.08 K can be reproduced to 0.00 K (0.02 K) in calculations using only 37 (27) reference configurations, built from the atomic 1s, 2s, 2p, and 3s natural orbitals. Using a very large basis, the 37‐reference set gives a best binding energy of −10.87 K, in satisfactory agreement wit...

Gradients in the ab initio scalar zeroth-order regular approximation (ZORA) approach

We discuss ways to obtain analytical gradients within the scalar zeroth-order regular approximation ZORA to the Dirac-Fock equation within an ab initio context. Simply employing the relativistic density within the non-relativistic gradient package is in error by 10 y5 . We introduce a new strictly atomic scheme which in addition to yielding exact gradients is also computationally inexpensive and avoids the gauge invariance problems that plague molecular ZORA approaches. We show that the total and orbital energies produced with the scaled version of this method are generally, i.e. except for very short interatomic distances, very close to the full molecular scaled ZORA results. Equilibrium geometries ˚ from full molecular scaled ZORA and strictly atomic ZORA are shown to be within 0.01 A from Dirac-Fock. q 2000 Published by Elsevier Science B.V.

Parallelism in computational chemistry: I. Hypercube-connected multicomputers

SummaryAn account is given of experience gained in implementing computational chemistry application software, including quantum chemistry and macromolecular refinement codes, on distributed memory parallel processors. In quantum chemistry we consider the coarse-grained implementation of Gaussian integral and derivative integral evaluation, the direct-SCF computation of an uncorrelated wavefunction, the 4-index transformation of two-electron integrals and the direct-CI calculation of correlated wavefunctions. In the refinement of macromolecular conformations, we describe domain decomposition techniques used in implementing general purpose molecular mechanics, molecular dynamics and free energy perturbation calculations. Attention is focused on performance figures obtained on the Intel iPSC/2 and iPSC/860 hypercubes, which are compared with those obtained on a Cray Y-MP/464 and Convex C-220 minisupercomputer. From this data we deduce the cost effectiveness of parallel processors in the field of computational chemistry.

Size consistent multireference single and double excitation configuration interaction calculations. The multireference coupled electron-pair approximation

A new size consistent extension to the multi reference configuration interaction method is described. The method termed multireference coupled electron pair approximation (MRCEPA) is akin to a multireference CEPA(0) approach, though nonlinear terms do receive separate attention. We show the performance of the approach in some model systems as well as in an application to calculation of ground and excited ππ* states of ethylene.

An ab initio two-component relativistic method including spin- orbit coupling using the regular approximation

In this paper we present the implementation of the two-component scaled zeroth-order regular approximation (ZORA) method in the molecular electronic structure package GAMESS-UK. It is the first application of this method, which was earlier investigated in the context of density functional theory, in molecular ab initio basis set calculations. The performance of the method is tested in atomic calculations, which we can compare with numerical results, on xenon and radon and in molecular calculations on the molecules AgH, HI, I2, AuH, TlH, and Bi2. In calculations on the I2 molecule we investigated the effect of the different approaches regarding the internal Coulomb matrix used in the ZORA method. For the remaining molecules we computed harmonic frequencies and bond lengths. It is shown that the scaled ZORA approach is a cost-effective alternative to the Dirac–Fock method.

Magnetic properties and aromaticity of o-, m-, and p-benzyne

The relative aromaticities of the three singlet benzyne isomers, 1,2-, 1,3-, and 1,4-didehydrobenzenes have been evaluated with a series of aromaticity indicators, including magnetic susceptibility anisotropies and exaltations, nucleus-independent chemical shifts (NICS), and aromatic stabilization energies (all evaluated at the DFT level), as well as valence-bond Pauling resonance energies. Most of the criteria point to the o-benzyne<m-benzyne<p-benzyne aromaticity order, whereas the relative aromaticity of each isomer with respect to benzene depends on the aromaticity criterion. An additional aromaticity evaluation involved the transition state of the Bergman cyclization of (Z)-hexa-1,5-diyn-3-ene which yields p-benzyne. Dissected NICS calculations reveal an aromatic transition state with a larger total NICS but a smaller NICS(pi) component and thus lower aromaticity than benzene.