[gmx-developers] Gromacs-FEP proposal

[Yuzhi Zhang]

Dear Gromacs developers, 
    Based on Gromacs-2020.2, we've done a series of development related to FEP in following aspects . We would like to contribute our codes to Gromacs and we're looking forward to hearing your opinions first.
    High-performance implementations: 
    1.  Offload FEP-PME to GPU.  Our status: finished. We've also noticed that this function has been implemented in commit f7be07e3cc901eb03700d93248fc09b573370282 by M. Lundborg et. al. and has been merged in Gromacs-2021. So we won't commit this. 
    2.  Offload non-bonded free energy kernel to GPU. Our status: finished. 
    3.  Offload bonded free energy kernel to GPU. Our status: finished.
    4.  Support GPU updates for FEP. Our status: finished md-integrator. We've noticed that this has also been implemented in Gromacs-2021, but we'are still interested in supporting GPU updates of sd-integrator.  Similar discussion can be found in issue  #3258 on gitlab. 
    5.  Support GPU replica exchange. Our status: finished.


    New features: 
    1.  Support "Soft Bond Potential" in FEP, which is proposed in  DOI: 10.1021/acs.jctc.6b00991 and allows a more smooth change of bond topology when there is bond formation or breaking in a FEP transformation, such as 5-member ring to 6-member ring.  Our status: finished. 
    2.  Split softcore parameters for VDW and Coulomb interactions.  We noticed that currently Gromacs supports only one parameter "sc-alpha" to modify soft-core interactions.  After our tests, this will cause a phase-transition-like phenomenon at middle lambdas in some cases.  Our status: finished.  We also noticed recently there have been some new forms of softcores  like gapsys softcore (DOI:  dx.doi.org/10.1021/ct300220p and discussed in https://gitlab.com/gromacs/gromacs/-/merge_requests/882 ) and Amber SSC2 softcore (DOI: 10.1021/acs.jctc.0c00237.)  And we would like to ask if you consider it's necessary to support such new softcore potentials. 
    These terms are all that we want to contribute to Gromacs. But we have to admit that there is still a lot of work to test and standardize our codes to meet all requirements of Gromacs.  Is there any suggestion? 


    Best wishes!
    Happy Simulating!

    Yuzhi & Junhan 
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