Unknown fix style


I have a question. When I want to use “fix shake” for SPC/E water model I got this error as

A small part of input:

group ox type 1
group hy type 2
group ca type 3

fix 2 all nvt temp 300.0 300.0 0.05
fix 3 hy all shake 0.0001 20 0 b 2 a 2

ERROR: Unknown fix style (…/modify.cpp:762)

When I checked “src” directory through the command bellow I found that the RIGID and SHAKE are available. But, I dont know what is happening.

lmp_serial -h

Could anybody help me to fix this problem?

Thanks in advance

fix shake style you used is not correct:


fix **ID** **group-ID** shake tol iter N constraint values ... keyword value ...

fix **3** **hy** **all** shake 0.0001 20 0 b 2 a 2

hy is extra!

Dear Xiaolin,

Thank you very much for you attention.Unfortunately, when I use the bellow commands I got the same error again.

fix 3 all shake 0.0001 20 0 b 2 a 2
fix 3 hy shake 0.0001 20 0 b 2 a 2

ERROR: Unknown fix style (…/modify.cpp:762)

Any suggestion is appreciated.


Did you install the RIGID package? If not, you have to install it and recompile. See http://lammps.sandia.gov/doc/Section_start.html#start_3

Dear Stefan,

Thank you for your response. I did. After installation I checked with “lmp_serial -h” command via “src” directory and I found that both shake and rigid are installed in “Fix styles”. I am really confused.

Any comments…

Can you check the LAMMPS log file? Maybe it is not shake but another fix causing the problem? In the log file you should find this error message right after the offending command.