Problem with fix SHAKE execution time

Dear Lammps users, I have a rather strange (to me) problem. When I use fix shake in my simulation to fix C-H bonds, the simulation slows down drastically (by an order of magnitude). However, “sometimes” the same simulation (yes, exactly the same input), runs very fast as expected. I don’t understand when this “sometimes” is. For example, if I submit the same script 20 different times, 2-3 times the script runs as expected and in all the other cases it is extremely slow.

The problem occurs only when I use fix shake in my input. I tried various random things like changing the order of fixes in the input, running on different no. of processers etc. and nothing seems to work so far.

Please help.

Additional details:
Simulation size is about 15000 atoms, I am using 20Jan15 version of Lammps. I use a couple of other fixes in my input, nvt and set force =0.0.

Thanks in advance.

Vikram Reddy Ardham
Graduate Student,
TU Darmstadt.

Sounds like a problem with your machine, not with LAMMPS.

What kind of platform are you running on? How many
cores/node? Are you locking processes to cores and not
allowing the OS to migrate them?

Also note, that with SHAKE you can typically run a 2x larger

(or more) timestep than w/out.