constraint algorithm

In computational chemistry, a constraint algorithm is a method for satisfying a Newtonian motion of a rigid which consists of mass points. By this algorithm, the distance between mass points is maintained constant. In general, the algorithm is constructed by following procedures; (i) choosing novel unconstrained coordinates (internal coordinates), (ii) introducing explicit constraint forces, (iii) minimizing constraint forces implicitly by the technique of Lagrange multipliers or projection methods.