A UNITARY-GROUP BASED OPEN-SHELL COUPLED-CLUSTER STUDY OF VIBRATIONALFREQUENCIES IN-GROUND AND EXCITED-STATES OF FIRST-ROW DIATOMICS

The performance of recently introduced coupled cluster (CC) method exp loiting the unitary group approach (UGA) to many-electron systems, tru ncated at the first order interacting space level [UGA-CCSD(is)] and u sing the 6-31G basis set, in computations of equilibrium bond lengths and harmonic vibrational frequencies, is examined for a series of ope n-shell (OS) states of the first row diatomics and hydrides. Altogethe r, 48 distinct electronic states are considered for 9 diatomic hydride s (BeH, BH, CH, CH+, NH, NH+, OH, OH+ and FH) and 18 diatomics (BeF, B N, BO, C-2, C-2(+), C-2(-), CN, CO, CO+, CF, N-2(+), NO, NO-, NF, O-2, O-2(+), OF and F-2(+)), involving both high 2 and low spin cases. Ver y good agreement with the available experimental data is found in all cases, except when the experimental values are marked as ''uncertain'' or where only the Delta G(1/2) values of harmonic frequencies are ava ilable. For the so-called ''difficult' systems, namely NO(X (2) Pi), O -2(X (3) Sigma(g)(-)), O-2(+)(X (2) Pi(g)), OF (X (2) Pi) and F-2(+) ( X (2) Pi(g)), the geometries and vibrational frequencies are also calc ulated using the TZ2P [5s4p2d] basis sets, and the results are compare d with both the experiment and existing perturbation theory and CC res ults. All results indicate that UGA CCSD(is) represents a versatile, r eliable and computationally affordable method that can handle a great variety of OS states, including OS singlets. (C) 1996 American Institu te of Physics.