Delano P. Chong

Interpretation of the Kohn-Sham orbital energies as approximate vertical ionization potentials

Theoretical analysis and results of calculations are put forward to interpret the energies −ek of the occupied Kohn–Sham (KS) orbitals as approximate but rather accurate relaxed vertical ionization potentials (VIPs) Ik. Exact relations between ek and Ik are established with a set of linear equations for the ek, which are expressed through Ik and the matrix elements ekresp of a component of the KS exchange-correlation (xc) potential vxc, the response potential vresp. Although −Ik will be a leading contribution to ek, other Ij≠k do enter through coupling terms which are determined by the overlaps between the densities of the KS orbitals as well as by overlaps between the KS and Dyson orbital densities. The orbital energies obtained with “exact” KS potentials are compared with the experimental VIPs of the molecules N2, CO, HF, and H2O. Very good agreement between the accurate −ek of the outer valence KS orbitals and the corresponding VIPs is established. The average difference, approaching 0.1 eV, is about a...

A modified coupled pair functional approach

A modified coupled pair functional (MCPF) method is presented that dramatically improves properties for cases where the Hartree–Fock reference configuration is not a good zeroth‐order description of the wave function. This new methodology is compared to singles‐plus‐doubles configuration interaction and the CPF method of Ahlrichs et al. for the ground states of NiH, CuH, and ZnH.

Theoretical dipole moments for the first‐row transition metal hydrides

Spectroscopic parameters (De, re, μ) are determined for the first‐row transition metal hydrides using better than DZP basis sets at the modified coupled pair functional (MCPF) level. Extensive comparisons between MCPF and complete‐active space self‐consistent field (CASSCF)/MRCI calculations with natural orbital iterations, and studies with more extensive basis sets, show this level of treatment to supply an accurate and cost‐effective treatment of these systems. For the transition metal hydrides, the bonding can arise from either the 3dn4s2 or 3dn+14s1 atomic asymptotes, or a mixture of both. Since the dipole moment arising from these two bonding mechanisms is very different, the dipole moment is found to be directly related to the 3d population. Thus, the magnitude of the dipole moments provide a sensitive test of the wave function, and gives insight into the nature of the bonding.

Finite-field Calculations of Molecular Polarizabilities Using Field-Induced Polarization Functions: Second- and Third-order Perturbation Correlation Corrections to the Coupled Hartree-Fock Polarizability of H2O

Ordinary Rayleigh-Schrodinger perturbation theory with Moller-Plesset (RSMP) partitioning is used to calculate second- and third-order correlation corrections to the CHF polarizability and dipole moment of the water molecule by a finite-field procedure. [2/1] Pade approximants are found to be useful in accelerating the convergence of the property perturbation expansions. Field-induced polarization functions suitable for polarizability calculations are determined. The average polarizability calculated, neglecting vibrational averaging, with Dunnings (9s5p/4s-4s2p/2s) contracted GTO basis set augmented by field-induced 1s1p2d/1p polarization functions is within 3 per cent of the experimental result. Correlation corrections to the dipole moment and polarizability of the water molecule calculated by the finite-field RSMP and single + double excitation CI(SDCI) methods for the same basis set are found to be in close agreement. The RSMP approach has the advantages of being size-consistent and of being capable ...

DENSITY-FUNCTIONAL CALCULATION OF CORE-ELECTRON BINDING ENERGIES OF C, N, O, AND F

The unrestricted generalized transition‐state model using B88/P86 functional with Dunning’s cc‐pV5Z basis set, found to be an excellent method of calculating core‐electron binding energies (CEBEs), was further applied to many more molecules, some of which contain atoms from the third period. Estimation of relativistic corrections has also been refined. The average absolute deviation of over 50 calculated CEBEs from experiment is 0.30 eV before inclusion of approximate relativistic corrections (Crel), and 0.23 eV after adding Crel. Those molecules with observed CEBEs served to confirm our procedure, whereas the other cases provided our prediction of CEBEs.

Comparison of local‐density and Hartree–Fock calculations of molecular polarizabilities and hyperpolarizabilities

This paper presents a comparison between density functional theory local density approximation (LDA) and Hartree–Fock approximation (HFA) calculations of dipole moments, polarizabilities, and first hyperpolarizabilities, using ‘‘comparable’’ basis sets, in order to assess the relative quality of the LDA and the HFA for calculating these properties. Specifically, calculations were done using basis sets of roughly double or triple zeta plus polarization quality, with and without added field‐induced polarization (FIP) functions, for the seven small molecules H2, N2, CO, CH4, NH3, H2O, and HF, using the HFA option in the program HONDO8 and the LDA options in the programs DMol and deMon. For the calculations without FIP functions, the results from HONDO8 HFA and deMon LDA, both of which use Gaussian basis sets, are very similar, while DMol, which uses a LDA numerical atomic orbital basis set, gives substantially better results. Adding FIP functions does much to alleviate these observed basis set artifacts and ...

Even-tempered Slater-type orbitals revisited: from hydrogen to krypton.

Even‐tempered Slater‐type orbital basis sets were developed in 1973, based on total atomic energy optimization. Here, we revisit ET STOs and propose new sets based on past experience and recent computational studies. From preliminary atomic and molecular tests, these sets are shown to be very well balanced and to perform, at lower cost, almost as well as a very large (close to complete) basis set.

Assessment of Gaussian-weighted angular resolution functions in the comparison of quantum-mechanically calculated electron momentum distributions with experiment

Abstract A critical assessment is made of the recently proposed momentum-averaged Gaussian-weighted (MAGW) method [A.O. Bawagan and C.E. Brion, Chem. Phys. 144 (1990) 167] of incorporating angular (or momentum) resolution into quantum-mechanically calculated momentum distributions for comparison with EMS measurements. In particular, the general efficacy of the proposed, semi-emperically based Gaussian angular (θ and ϕ) resolution functions as dimensioned in the MAGW method is tested by systematic application to high-level (essentially Hartree-Fock limit and/or configuration interaction) calculations of the momentum distributions of the outermost orbitals of a wide range of target species including Ne, Ar, Kr, Xe, H2O, and H2S. The folded calculations are compared with recent measurements. New highly accurate analytical mathematical procedures have been developed and confirm the adequacy of the previous Monte Carlo method for resolution folding. However, the new procedures result in substantial improvements over use of the Monte Carlo method for the graphical representation of the variation of the momentum resolution function with azimuthal angle. The respective merits of comparing folded theory and experiment as a function of nominal relative azimuthal angle (ϕ0) or of momentum are discussed. In the momentum representation, the question of whether it is preferable to use average or nominal momentum is further considered. Either choice is found to afford a reasonable basis for detailed comparison of folded theory and experiment. In all cases, the folded theory is found to be in good agreement with experiment when the Gaussian-weighted angular resolution functions are used, whereas less satisfactory overall agreement is obtained when the calculations are folded with earlier types of resolution-folding procedure. It is concluded that the Gaussian-weighted angular resolution functions, appropriately dimensioned, provide a satisfactory accounting for the experimental angular (or momentum) resolutions effects and that they can be used with reasonable confidence in future studies for the evaluation of molecular wavefunctions.

Accurate calculation of core-electron binding energies by the density-functional method

Abstract Using a basis set of atomic natural orbitals of valence triple-zeta plus double polarization quality, the core-electron binding energies (CEBEs) of HF, H2O, N2 and CO were computed by the deMon density-functional program with a combined functional of Beckes exchange (B88) with Perdews correlation (P86). Of four different versions of the transition-state method tested, the unrestricted generalized transition-state (uGTS) model showed most promise. Two other functionals were tested but found to be inferior to the B88/P86 combination. The molecules HCN, CO2 and H2CO were added in the comparative study of basis set convergence. The best procedure found (the uGTS model using B88/P86 functional with Dunnings cc-pV5Z basis set) was applied to other molecules. The average deviation from experiment for 20 computed CEBEs is 0.23 eV.

Theoretical study of polarizabilities and hyperpolarizabilities of Ne, HF, F−, and OH−

Theoretical calculations are presented for the polarizability parameters through the fourth power of an applied static electric field for Ne and HF and through the second power in the field for the negative ions F− and OH−. Large Gaussian basis sets are employed and electron correlation is incorporated using the modified coupled pair functional (MCPF) method and for Ne and HF using coupled‐cluster single‐ and double‐excitation (CCSD) methods with two approaches of including the effect of triple excitations. The MCPF method is found to perform well in relation to the more rigorous CCSD method. Our theoretical values for Ne are in good accord with the recent experimental value of Shelton, as well as with recent theoretical studies. Our best results for the first and second hyperpolarizability of HF are in slightly better agreement with experiment than previous theoretical studies, but still show a very large disparity.