Negative Frame number in trajectory

User discussions
1

GROMACS version: 2022.5
GROMACS modification: No

Hi,

I have a small quick question. I am running 100 ns simulation on a protein-ligand complex in POPC bilayer. My system has been made on CHARMM GUI. I was using gmx gromp to make tpr file for one of the NPT equilibrations. When I used the following command:

gmx grompp -f step6.6_equilibration.mdp -c npt3.gro -r npt3.gro -t npt3.cpt -p topol.top -n index.ndx -o npt4.tpr

I got the following output:

Reading Coordinates, Velocities and Box size from old trajectory

Will read whole trajectory
Last frame -1 time 500.000

Using frame at t = 500 ps

I understand that Gromacs has 0 based indexing where first frame is 0, second is 1 and so on. I am confused why last frame is printed as -1. Is this a normal behavior for Gromacs?

Can anyone shed some light on this issue.

Thanks in advance

2

Dear @Ligand1,

It should be okay, I think -1 is the standard notation to say ‘independently from the number of frames, take the last one’, a bit like if you were to take the last element of a python array, in case you are familiar with python coding.

3

Many thanks for your answer.

Best Regards