THE EFFECTIVENESS OF NEWTONS METHOD FOR IMPROVING AB-INITIO FORCE-FIELDS WITH APPLICATIONS TO CO2 AND H2CO

Ab initio force field parameters are refined using modifications to Ne wton's method in which a figure of merit function is approximated as a Taylor Series truncated at second order. We investigate two versions of Newton's method: the Discrete Newton method where the Hessian is ap proximated by finite differences and the Gauss-Newton method where the Hessian is approximated as the product of first derivatives. The appl icability of both rests on the capability of current nb initio methods to calculate quartic force fields that accurately reproduce experimen tal observables such as band centers and inertial constants. As exampl es, we calculate refined potential energy surfaces for CO2 and H2CO. W e show that an algorithm depending solely on Newton's methods requires only a small number of iterations. Additionally, Newton-based methods provide a great deal of information about the sensitivity of the forc e field parameters to the observables being fit. We also demonstrate t hat the less computationally demanding Gauss-Newton method gives resul ts similar to the less approximate Discrete Newton method. (C) 1998 Ac ademic Press.