Full configuration interaction potential energy curves for the X 1Σg+, B 1Δg, and B′ 1Σg+ states of C2: A challenge for approximate methods

摘要: The C2 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 X 1Σg+, B 1Δg, B′ 1Σg+ states C2, which exactly solve Schrodinger equation within space spanned by 6-31G* basis set. Within D2h subgroup used most structure programs, these all have same symmetry (1Ag), three become energetically close interatomic distances beyond 1.5 A. quality single-reference ab initio methods is assessed comparison to benchmark results. Unfortunately, even coupled-cluster theory through perturbative triples using an unrestricted Hartree–Fock reference large nonparallelity errors (>20 kcal mol−1) ground state. are not accurately modeled any commonly method, nor interac...