Molecular mechanics valence bond methods for large active spaces. Application to conjugated polycyclic hydrocarbons

Abstract The implementation of the Clifford-algebra unitary group approach (first discussed by Paldus) in the context of a Heisenberg Hamiltonian formulation of the VB problem is described. It is demonstrated that this technique leads to an efficient extension of the molecular mechanics valence bond (MM-VB) method to large active space applications, by carrying out full geometry optimisations on the S0 state of coronene (24 active electrons), the S0 and S1 states of naphthalene (10 active electrons) and the S0 and T1 states of triangulene (22 active electrons).