hi tim

it depends on the type of intramolecular term in the force-field definition. if the terms are such that the bond lengths or angles are flexible (e.g. harmonic potential terms), then, as far as i know, there is no limit to the number of such terms in the force-field definition.

however, if the intramolecular terms are rigid constraints, such as those applied by the SHAKE algorithm, then there is a limit to the number of such constraints, namely the total number of degrees of freedom in the molecule (3N in three dimensions, where N is the number of atoms in the molecule). of course, if you applied 3N constraints, your molecule wouldn’t be able to move at all.

for the simple case of a non-flexible water molecule, for which the total number of degrees of freedom is 3*3 = 9, 2 bond length constraints and 1 bond angle constraint are applied (or, equivalently, 3 bond length constraints - 2 “real” bonds and 1 “artificial” bond between the hydrogen atoms), leaving the molecule with 6 degrees of freedom. These 6 degrees of freedom correspond to the 3 translational and 3 rotational degrees of freedom of the molecule as a whole. in general, for a non-linear molecule, the maximum total number of rigid constraints that can be applied (so that the molecule can still rotate and translate) is 3N-6. for a linear molecule, it is 3N-5.

-david