Timehyphen;dependent coupled cluster approach: Excitation energy calculation using an orthogonally spinhyphen;adapted formalism

摘要

In the timehyphen;dependent coupled cluster approach the excitation energies can be obtained as eigenvalues of a generally nonhyphen;Hermitian matrix characterizing the homogeneous part of a linear response equation. This matrix consists of the Hermitian configuration interaction part as well as of a nonhyphen;Hermitian part which describes the lsquo;lsquo;renormalizationrsquo;rsquo; due to ground state correlations. The relationship between a singular behavior of the ground state coupled cluster equations and the existence of vanishingly small excitation energies in the linear response formalism is pointed out. The explicit form of the required matrix elements in the orthogonally spinhyphen;adapted version of the theory is given for configuration spaces restricted to single and double excitations. A numerical test of the formalism for pgr;hyphen;electron models oftranshyphen;butadiene and benzene is carried out and the relationship of the timehyphen;dependent coupled cluster approach with other many body approaches to excitation energies calculation is discussed.