FULL VARIATIONAL MOLECULAR-ORBITAL METHOD - APPLICATION TO THE POSITRON-MOLECULE COMPLEXES

Optimal Gaussian-type orbital (GTO) basis sets of positron and electro n in positron-molecule complexes are proposed by using the full variat ional treatment of molecular orbital (FVMO) method. The analytical exp ression for the energy gradient with respect to parameters of positron ic and electronic GTO such as the orbital exponents, the orbital cente rs, and the Linear combination of atomic orbital (LCAO) coefficients, is derived. Wave functions obtained by the FVMO method include the eff ect of electronic or positronic orbital relaxation explicitly and sati sfy the virial and Hellmann-Feynman theorems completely. We have demon strated the optimization of each orbital exponent in various positron- atomic and anion systems, and estimated the positron affinity (PA) as the difference between their energies. Our PA obtained with small basi s set is in good agreement with the numerical Hartree-Fock result. We have calculated the OH- and [OH-; e(+)] species as the positron-molecu lar system by the FVMO method. This result shows that the positronic b asis set not only becomes more diffuse but also moves toward the oxyge n atom. Moreover, we have applied this method to determine both the nu clear and electronic wave functions of LiH and LiD molecules simultane ously, and obtained the isotopic effect directly. (C) 1998 John Wiley & Sons, Inc.