Ab initio molecular electronic structure theory has been used in an attempt to characterize the low-lying stationary points on the Hs+ potential energy hypersurface. Three distinct levels of theory have been used: the self-consistent-field (SCF) method, configuration interaction (CI) including all single and double excitations, and full configuration interaction. Four different basis sets were used: double zeta (DZ), double zeta plus polarization (DZP), an extended basis set designated H (6s2pI4s2p), and a second extended basis set designated H (8s3pl6s3p).The higher levels of theory are in agreement that the only minimum for Hs+ is a C Zv structure, with three other stationary points (of D 2d , C 2u ' and DZh symmetries) lying less than 1 kcall mol higher in energy. The predicted dissociation energy Do is 5.5 kcallmol, which is estimated to be about 1 kcallmolless than the exact Do. Furthermore, there are six other stationary points lying less than 8 kcallmol above the minimum. Vibrational frequencies, dipole moments, and infrared intensities for each of the ten stationary points have been predicted at several different levels oftheory. From the perspective of quantum chemistry, the Hs+ system is very attractive as a candidate for the study of the vibrational dynamics of weakly bound systems.
Equations are presented for the analytic determination of dipole moment derivatives with respect to nuclear coordinates for closed-shell, open-shell unrestricted, and open-shell restricted Hartree–Fock wave functions. The efficient evaluation of these derivatives and the resulting infrared intensities simultaneously with determination of the vibrational frequencies is discussed. Intensities are presented for a selection of test molecules with a wide variety of basis sets. It is concluded that basis sets of double-zeta polarized or higher quality usually give correct qualitative information about the ordering of the intensities, while smaller basis sets may not even predict the most intense mode correctly. Quantitative accuracy using the larger basis sets seems to be limited primarily by the use of the double harmonic approximation.
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