ELECTRONIC-STRUCTURE AND PROPERTIES OF STRAINED POLYMERS .1. METHODS FOR POLYETHYLENE

The modulus and strain-dependent frequency spectrum of polyethylene have been computed from a cluster of C6H12 units using the semi-empirical AM1 Hamiltonian and unrestricted Hartree-Fock wavefunctions. The computed modulus substantially exceeds experimental values, a result which derives from neglect of electron correlation ih the calculations. Good qualitative and quantitive agreement with reported strain-dependent frequencies is found, suggesting that the method describes cubic anharmonicity in the bonding while systematically overestimating components of the Hessian.