Full configuration interaction potential energy curves for the X 1∑ g+,B 1Δ g, and B′ 1∑ g+ states of C 2: A challenge for approximate methods

Abstract

The C 2 molecule exhibits unusual bonding and several low-lying excited electronic states, making the prediction of its potential energy curves a challenging test for quantum chemical methods. We report full configuration interaction results for the X 1∑ g+, B 1Δ g, and B′ 1∑ g+ states of C 2, which exactly solve the electronic Schrödinger equation within the space spanned by a 6-31G* basis set. Within the D 2h subgroup used by most electronic structure programs, these states all have the same symmetry ( 1A g), and all three states become energetically close for interatomic distances beyond 1.5 Å. The quality of several single-reference ab initia methods is assessed by comparison to the benchmark results. Unfortunately, even coupled-cluster theory through perturbative triples using an unrestricted Hartree-Fock reference exhibits large nonparallelity errors (>20kcalmol -1) for the ground state. The excited states are not accurately modeled by any commonly used single-reference method, nor by configuration interaction including full quadruple substitutions. The present benchmarks will be helpful in assessing theoretical methods designed to break bonds in ground and excited electronic states. © 2004 American Institute of Physics.