# How to find equilibrum time in MD?

**URL:** <https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206>\
**Category:** User discussions\
**Created:** [December 5, 2020, 6:21pm UTC](https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206 "2020-12-05T18:21:43Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![mj.rezayani](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/mj.rezayani/32/3918_2.png) [@mj.rezayani](https://gromacs.bioexcel.eu/u/mj.rezayani)\
**Post date:** [December 5, 2020, 6:21pm UTC](https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206/1 "2020-12-05T18:21:43Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Here post your question  
Dear all

I am simulation water tip4p diffusion in polymer. I want to be sure that my system has reached equilibrium state. Actually, I want to know after what time the analysis is reliable to get a suitable time for simulation? What kind of analysis can be used for this purpose?

Thank you very much

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**Author:** ![Masrul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/masrul/32/234_2.png) [@Masrul](https://gromacs.bioexcel.eu/u/Masrul)\
**Post date:** [December 5, 2020, 6:35pm UTC](https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206/2 "2020-12-05T18:35:27Z")

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First you need to make sure system’s temperature, pressure and energies are equilibrated, which can be checked using **gmx energy**. Plot these properties, see if you reached equilibrium. In addition to these, you might want to check **radius of gyration vs. time** convergence for polymeric system.

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**Author:** ![mj.rezayani](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/mj.rezayani/32/3918_2.png) [@mj.rezayani](https://gromacs.bioexcel.eu/u/mj.rezayani)\
**Post date:** [December 5, 2020, 8:46pm UTC](https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206/3 "2020-12-05T20:46:14Z")

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Dear Masrul

Thank you very much for your response. I have another question.Does Velocity Autocorrelation Function provide good results for prediction of equilibrium? I heard when it converges to zero we can grantee  
equilibrium state?

Thank you again for your reply

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**Author:** ![Masrul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/masrul/32/234_2.png) [@Masrul](https://gromacs.bioexcel.eu/u/Masrul)\
**Post date:** [December 6, 2020, 8:52pm UTC](https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206/4 "2020-12-06T20:52:20Z")

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Hello,

I never used velocity autocorrelation to determine equilibrium state, I can’t comment. Velocity autocorrelation can be used to calculate dynamic properties such as, diffusion coefficient, viscosity.

Regards,  
Masrul

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**Author:** ![mj.rezayani](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/mj.rezayani/32/3918_2.png) [@mj.rezayani](https://gromacs.bioexcel.eu/u/mj.rezayani)\
**Post date:** [December 6, 2020, 9:50pm UTC](https://gromacs.bioexcel.eu/t/how-to-find-equilibrum-time-in-md/1206/5 "2020-12-06T21:50:58Z")

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Hello dear Masrul

Thank you very much for your reply.

Best