Based on the experimental structures and vibrational spectra of eight alkylphosphines, as well as ten ab initio calculated (MP2/6-31+g*) structures and five potential energy profiles, the MM4 force field has been extended to include this important functional group. The results are comparable to experimental values for phosphorus-containing compounds. The addition of various cross-terms significantly improved the MM4 calculated structures relative to its predecessor, MM3. The overall root mean square error in moments of inertia have been reduced from 1.65% in MM3 to 0.26% in MM4. MM4 was also successful in reducing the previously impressive root mean square error in vibrational frequencies from 35 cm(-1) to 31 cm(-1) for the eight compounds studied.
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