# What is the standard state of free energy calculations on GROMACS?

**URL:** <https://gromacs.bioexcel.eu/t/what-is-the-standard-state-of-free-energy-calculations-on-gromacs/7210>\
**Category:** User discussions\
**Tags:** free-energy\
**Created:** [September 18, 2023, 8:46am UTC](https://gromacs.bioexcel.eu/t/what-is-the-standard-state-of-free-energy-calculations-on-gromacs/7210 "2023-09-18T08:46:20Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![horlust](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@horlust](https://gromacs.bioexcel.eu/u/horlust)\
**Post date:** [September 18, 2023, 8:46am UTC](https://gromacs.bioexcel.eu/t/what-is-the-standard-state-of-free-energy-calculations-on-gromacs/7210/1 "2023-09-18T08:46:20Z")

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GROMACS version: 2023.2  
GROMACS modification: No

Hi everyone!

I am new to GROMACS. I am calculating free energies of solvation for some molecules in water through the BAR algorithm. I wanted to use this data to calculate activity coefficients, but I can’t seem to find information on what is the standard state of this calculation (I suppose the calculated deltaG is the difference between the free energy of the state and of the standard state). My best guesses are ideal gas at 1M or at 1 bar (the pressure I adopt throughout my calculations). How can I be sure of this?

Thank you very much :-)

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**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [September 18, 2023, 9:59am UTC](https://gromacs.bioexcel.eu/t/what-is-the-standard-state-of-free-energy-calculations-on-gromacs/7210/2 "2023-09-18T09:59:28Z")

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The standard state is a single molecule in vacuum.

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<div class="post-metadata">

**Author:** ![horlust](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@horlust](https://gromacs.bioexcel.eu/u/horlust)\
**Post date:** [September 18, 2023, 10:06am UTC](https://gromacs.bioexcel.eu/t/what-is-the-standard-state-of-free-energy-calculations-on-gromacs/7210/3 "2023-09-18T10:06:08Z")

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Thank you. I was confused because according to some articles I read the reported values are for transfer free energies from gas (at 1M standard state) to solution (at 1M standard state). And from what I could understand they were doing the same calculations as me (similar .mdp files). But I agree with you, it makes sense that it is a single molecule in vacuum.

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<div class="post-metadata">

**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [September 18, 2023, 10:58am UTC](https://gromacs.bioexcel.eu/t/what-is-the-standard-state-of-free-energy-calculations-on-gromacs/7210/4 "2023-09-18T10:58:18Z")

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You can’t have 1M in the gas state. Molality involves a solvent.