Protein getting out of water box

GROMACS version: 2020.02
GROMACS modification: Yes/No
I have done a 500ns simulation of a protein (256aa) using Gromacs. While analyzing in VMD, I found that the protein gets slightly out of the box after 420ns. Also, the RMSD score seems to increase after about 350ns. Why is this happening?

I have used AMBER99SB-ILDN forcefield. Padding distance was 1.1 and box volume was 512 nm3.

Most likely http://manual.gromacs.org/documentation/current/user-guide/terminology.html#gmx-pbc

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