A new procedure for refining force constants in normal coordinate calculations on large molecules

We develop a force‐constant refinement procedure which we believe capable of being used in problems of large molecules and biopolymers. The procedure is based on a Green‐function expression which relates changes in frequency to changes in force constants. The method does not require that assignments be made before refinment (although they may be). An expansion of this expression gives rise to a set of linear algebraic equations for the force‐constant corrections, rather than an equation involving residuals to minimize. The resulting calculations are considerably simpler. This approximate is iterated to find the final refined force‐constants. We discuss several methods of improving the convergence of the procedure which take into account the experimental information which may be available. Of particular interest is a scheme to select the force constants to be refined by imposing a criterion for selecting those which will fit the experimental data with the smallest changes of the force constants from their expected values. We discuss some limitations which occur for all methods of refinement applied to large molecules.