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: <a rel="noopener">10.1103/PhysRevB.98.224301</a>, <a rel="noopener">10.1103/PhysRevLett.121.195701</a>), the common approach is: <ol> <li> Take an ideal gas as the reference state (whose free energy can be calculated analytically). </li> <li> Then, via Hamiltonian Thermodynamic Integration (H-TI), gradually switch to the real system described by ab initio methods (e.g., DFT). </li> </ol> When trying to apply this workflow in CP2K, I have several specific questions: <hr /> 1) Realization of the ideal gas reference state <ul> <li> 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)? </li> <li> If so, would such a zero-interaction MM/FIST system be a reasonable reference state for TI? </li> </ul> <hr /> 2) Choice of initial configuration and box size <ul> <li> 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? </li> <li> More generally, are there any special considerations in CP2K regarding the initial configuration and box size when using an ideal-gas reference state? </li> </ul> <hr /> 3) Thermodynamic integration from ideal gas → ab initio system <ul> <li> Is it possible to directly perform H-TI from the ideal gas reference state to the DFT/Quickstep description within CP2K? </li> <li> 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? </li> </ul> <hr /> 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! -- You received this message because you are subscribed to the Google Groups "cp2k" group. To unsubscribe from this group and stop receiving emails from it, send an email to <a href="mailto:cp2k+unsubscribe@googlegroups.com">cp2k+unsubscribe@googlegroups.com</a>. To view this discussion visit <a href="https://groups.google.com/d/msgid/cp2k/90afa236-aeef-4e77-a31e-0b83b2576a90n%40googlegroups.com?utm_medium=email&utm_source=footer">https://groups.google.com/d/msgid/cp2k/90afa236-aeef-4e77-a31e-0b83b2576a90n%40googlegroups.com</a>.