Approximate spin-projected spin-unrestricted density functional theory method: Application to the diradical character dependences of the (hyper) polarizabilities in p-quinodimethane models

To demonstrate the performance of a novel approximate spin-projected (ASP) spin-unrestricted density functional theory (UDFT) method, we examine the diradical character dependences of the static polarizability (a) and second hyperpolarizability (c) of a model open-shell singlet molecule, p-quinodimethane (PQM), using several hybrid and long-range corrected (LC) exchange-correlation functionals. The ASP-LC-UBLYP method with a range-separating parameter mu = 0.47 is found to reproduce semi-quantitatively the diradical character dependences of alpha and gamma of the PQM model calculated using the strongly-correlated UCCSD(T) method. (C) 2010 Elsevier B.V. All rights reserved.