[CP2K-user] [CP2K:21792] The problem of achieving Thermodynamic Integration from ideal gases to ab initio systems in CP2K

[ice ZQing]

*Hello everyone,*

I have recently been learning and trying to use *Thermodynamic Integration 
(TI)* to calculate the free energy of systems. According to the book *Understanding 
Molecular Simulation: From Algorithms to Applications* (Frenkel & Smit) and 
some subsequent application papers (mainly based on VASP, e.g. DOI: 
10.1103/PhysRevB.98.224301, 10.1103/PhysRevLett.121.195701), the common 
approach is:

   1. 
   
   Take an *ideal gas* as the reference state (whose free energy can be 
   calculated analytically).
   2. 
   
   Then, via *Hamiltonian Thermodynamic Integration (H-TI)*, gradually 
   switch to the real system described by *ab initio* methods (e.g., DFT).
   
When trying to apply this workflow in CP2K, I have several specific 
questions:
------------------------------

*1) Realization of the ideal gas reference state*

   - 
   
   Can an “ideal gas” be represented in CP2K using the FIST/MM module, by 
   completely switching off all non-bonded interactions (so that the potential 
   energy of the system is always zero)?
   - 
   
   If so, would such a zero-interaction MM/FIST system be a reasonable 
   reference state for TI?
   
------------------------------

*2) Choice of initial configuration and box size*

   - 
   
   If the “zero potential energy” approach is adopted for the ideal gas, 
   does the simulation box need to be very large (so that particle 
   interactions are negligible), or is this unnecessary once all interactions 
   are explicitly switched off?
   - 
   
   More generally, are there any special considerations in CP2K regarding 
   the initial configuration and box size when using an ideal-gas reference 
   state?
   
------------------------------

*3) Thermodynamic integration from ideal gas → ab initio system*

   - 
   
   Is it possible to directly perform H-TI from the ideal gas reference 
   state to the DFT/Quickstep description within CP2K?
   - 
   
   Most of the literature I have seen uses VASP for this type of 
   calculation. Are there known examples, recommended workflows, or best 
   practices for implementing this in CP2K?
   
------------------------------

I have reviewed some literature but could not find a clear demonstration of 
how to set up the CP2K input and methodology for this case. I would greatly 
appreciate any advice, example input files, or references that could point 
me in the right direction.

*Thank you very much for your time and help!*

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