Introduction of Andersen’s demon in the molecular dynamics of systems with constraints

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Abstract
The method of constant pressure molecular dynamics (MD), developed by Andersen for monoatomic fluids is extended to the MD, in Cartesian coordinates, of molecular systems with constraints. Andersen’s proof is easily generalized after decoupling internal degrees of freedom from the center‐of‐mass. Only these last degrees are directly affected by Andersen’s transformation (demon). The Cartesian equations of motion of individual atoms are derived from a generalized Andersen’s Lagrangian. The equations are quite similar to those of the usual MD simulation at constant volume apart from an additional term coupling the molecular center‐of‐mass and the volume of the sample. The volume appears now as a dynamical variable evolving from the imbalance between imposed external pressure and instantaneous values of the molecular stress tensor. Some numerical aspects are discussed and the technique is briefly illustrated for the case of rigid diatomic molecules.