[CP2K-user] [CP2K:19841] The treatment of atom mass in the mixing force_evals calculation

[Xu Wang]

Dear CP2K developers, 
    I am sorry to bother you for a technical detail regarding the treatment 
of atom mass in the mixing force_evals calculation.
    I intend to investigate the free energy difference between Na-Cl and 
K-Cl ion pairs in aqueous solutions through FPMD-based thermodynamic 
coupling constant integration, i.e., transitioning Na(+) gradually into 
K(+) using the &MIXED section:
[image: CP1.png]
together with the following settings in the &SUBSYS section:
[image: CP2.png]
    During the simulation, energies and forces are mixed via lambda to 
solve the equation of motion. On the other hand, the masses of atoms are 
also required in the equation of motion. I am wondering how CP2K handles 
the mass of “Na” mentioned above in simulations with 0<lambda<1. 
Specifically, is it calculated as 0.5*(mass_Na+mass_K), lambda*mass_Na+(1- 
lambda)*mass_K, or utilizing another algorithm? Thanks in advance!

Sincerely,
Xu

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