[gmx-users] Invalid Order for Directive Defaults Error Due to Force Field Mixing

[Sanim Rahman]

Hi all,

I am attempting to run a membrane protein simulation where I am describing
my protein and solvent in terms of CHARMM27 and my lipids in CHARMM36. When
I use the grompp command, I get the following error:

Fatal error:
Syntax error - File forcefield.itp, line 9
Last line read:
'[ defaults ]'
Invalid order for directive defaults

To create my topology file, I took my protein.top and at the bottom
included my lipid.itp and solvent.itp. The error is referencing to my
charmm36 force field file. I was unsure what to do so I just removed the
line with [ defaults ] on it and tried running it again. This time I
received this error:

Fatal error:
Syntax error - File ffnonbonded.itp, line 5
Last line read:
'[ atomtypes ]'
Invalid order for directive atomtypes

Is the source of the error from how my topology files are processing the
force field files since I am using both CHARMM27 and CHARMM36? I read a
previous thread that you can't have two [ default ] lines which make sense
but I am unsure of what is the proper protocol to get around this to
include both force fields. Any help will be deeply appreciated!

Thank You,

*Sanim Rahman*
B.S. Chemical Engineering, 2019
Resident Assistant, Castor Hall Engineering Living Learning Community
2016-2017
Co-Founder and Co-President of the Undergraduate Research Society
Undergraduate Researcher, Global Center for Hearing and Speech Research