A systematic exploration of the importance of cross terms in the Ni(IT) porphine force field is reported. Several force fields of varying complexity were generated using a modification of the Hessian-biased singular value decomposition (HBFF-SVD) approach originally developed by Goddard et al. The X-ray crystal structure, a B3LYP/6-31G(d,p) Hessian matrix, and experimental vibrational frequencies were used. The diagonal-only force field is inadequate for reproducing experimental frequencies. As anticipated, inclusion of 1,2 and 1,3 cross terms significantly improves results (total rms error 14.6 cm(-1); in-plane rms error = 12.0 cm(-1)). The longer range terms in a complete, in-plane, force field improve performance dramatically (in-plane rms error = 4.8 cm-1). A total of 83 long range interaction constants have values greater than or equal to 10 kcal/(mol geom-unit), and 5 are greater than 20 kcal/(mol geom-unit). For example, 1,6- and 1,9-(C-alpha-C-m)-(C-alpha-C-m) stretch- stretch, 1,4-(C-alpha-N)-(C-alpha-C-m), and 1,4-(C-beta-C-beta)-(C-alpha-N-C-alpha) stretch-bend interactions are large and positive. Coupling in- and out-of-plane motions (TORX) enhances out-of-plane accuracy. Though isotopomer data were omitted from the optimization, the HBFF-SVD force fields reproduce these data with high fidelity. Finally, the HBFF-SVD force fields are compared in detail to previous normal-mode analysis and scaled quantum mechanics studies of Ni porphine.