Revisiting particle dynamics in the NPT ensemble under the extended Lagrangian approach

Abstract

ABSTRACT Two main lines of research have been developed to simulate particle systems under thermodynamic conditions. The traditional line is based on the extended Lagrangian approach and the rigorous one is based on Lagrangian theory. The purpose of this work is to revisit the extended Lagrangian approximation because it is commonly employed to simulate a system of particles in the isobaric–isothermal ensemble. The extended Lagrangian approach introduces additional degrees of freedom (dof) to simulate the pressure and temperature effects on the particles. The equations of motion are of Newtonian type coupled to the extra dof. Yet, to avoid the large temperature and volume fluctuations, the Non-Hamiltonian approach is applied to create chains of thermostats and drive the system to the equilibrium state in a smooth and fast way. The thermodynamic conditions on the particle system are improved under such an approach. The pros and cons of the extended Lagrangian approach in combination with the Non-Hamiltonian method are discussed.