Showing papers by "Enrico Clementi published in 1991"

Electronic and vibrational properties of C60 at finite temperature from ab initio molecular dynamics.

Abstract: A Car-Parrinello molecular-dynamics study of ${\mathrm{C}}_{60}$ in the fullerene structure has been performed. The ground-state properties were calculated and found to be in agreement with other theoretical estimates. The finite-temperature properties of such clusters are computed. These can be directly compared to experiments performed at finite temperature. The comparison is very favorable for structural, vibrational, and electronic properties.

Dirac-Fock self-consistent field method for closed-shell molecules with kinetic balance and finite nuclear size

Abstract: We present a general method of implementing the kinetic balance condition within the Dirac-Fock (DF) self-consistent field (SCF) formalism for closed-shell molecular structure. We review the steps leading to the derivation of DF SCF equations for closed-shell molecules, particularly as formulated by Matsuoka et al. In the present approach, the large component of the molecular spinors are expanded in terms of atomic basis spinors of spherical-type Gaussian functions, with the small component related to the large component by the kinetic balance condition. It is shown that imposing the kinetic balance condition on geometric Gaussian-type basis functions allows us to obtain the Fock matrix elements, involving both the large and the small components, form the standard nonrelativistic Cartesian-type matrix elements. By using properties of orthogonal polynomials, the solid spherical harmonics are expressed in Cartesian form, thus providing a general basis for transformation of one- and two-electron-matrix elements, obtained from a Cartesian Gaussian-type basis, to a spherical Gaussian-type basis. The advantages of using kinetically balanced geometric Gaussian-type basis functions in molecular DF calculations including finite-size nucleus effects are emphasized. For the sake of completeness, we have added in an appendix corrections to the nuclear attraction matrix elements for the finite-size nucleus already derived by Matsuoka.

Gaussian functions in Hylleraas‐configuration‐interaction calculations. VI. The first excited state of H+3

Abstract: The Hylleraas‐configuration‐interaction (H‐CI) method has been applied to the first 3Σ+u excited state of the H+3 molecular ion. Besides the ground state, this is the only known bound state of H+3, albeit with a very weak minimum of 8.428 kcal/mol. The equilibrium geometry and the minimum energy have been determined. The Hylleraas‐CI energy of −1.116 102 7 a.u. at an equidistant nuclear separation of 2.454 bohr is the lowest reported so far. The vibrational force constants and vibrational frequencies also have been computed.

The Hylleraas-CI method in molecular calculations. III.: implementation and numerical verification of a three-electron many-center theory

Abstract: The general theory of three-electron Hylleraas-Configuration-Interaction method using linear correlation factors of the form rij has been implemented for molecular systems using cartesian Gaussians as basis sets. A brief review of the theory and the form of the three-electron integrals is presented. Additionally, a table of numerical values of some selected three-electron integrals is given. Results from test calculations on H3 using the full form of the theory are presented for some simple basis sets. A discussion of the computational problems that need to be overcome before this approach is competitive with traditional methods is included.

Two-electron integral evaluation for uncontracted geometrical-type Gaussian functions

Abstract: A new algorithm for efficient evaluation of two-electron repulsion integrals (ERIs) using uncontracted geometrical-type Gaussian basis functions is presented. Integrals are evaluated by the Habitz and Clementi method. The use of uncontracted geometrical basis sets allows grouping of basis functions into shells (s, sp, spd, or spdf) and processing of integrals in blocks (shell quartets). By utilizing information common to a block of integrals, this method achieves high efficiency. This technique has been incorporated into the KGNMOL molecular interaction program. Representative timings for a number of molecules with different basis sets are presented. The new code is found to be significantly faster than the previous program. For ERIs involving only s and p functions, the new algorithm is a factor of two faster than previously. The new program is also found to be competitive when compared with other standard molecular packages, such as HONDO-8 and Gaussian 86.