A benchmark study of molecular structure by experimental and theoretical methods: Equilibrium structure of thymine from microwave rotational constants and coupled-cluster computations

Accurate equilibrium, r(e), structures of thymine have been determined using two different, and to some extent complementary techniques. The composite ab initio Born-Oppenheimer, r(e)(best ab initio), structural parameters are obtained from the all-electron CCSD(T) and MP2 geometry optimizations using Gaussian basis sets up to quadruple-zeta quality. The semi-experimental mixed estimation method, where internal coordinates are fitted concurrently to equilibrium rotational constants and geometry parameters obtained from a high level of electronic structure theory. The equilibrium rotational constants are derived from experimental effective ground-state rotational constants and rovibrational corrections based on a quantum-chemical cubic force field. Equilibrium molecular structures accurate to 0.002 angstrom and 0.2 degrees have been determined. This work is one of a few accurate equilibrium structure determinations for large molecules. The poor behavior of Kraitchman's equations is discussed. (C) 2014 Elsevier B.V. All rights reserved.