Md energy minimization

GROMACS version: 2019.1
GROMACS modification: No

Dear All

I am running simulations of a protein in water and on energy minimizations, I want to run both Steepest Descent and Conjugate Gradient methods. Firstly, I am very new to MD! I initially ran a separate steepest descent energy minimization step and that was succesful, but on trying to run the cg method I am getting the following error:

Fatal error:
The coordinates could not be constrained. Minimizer ‘cg’ can not handle
constraint failures, use minimizer ‘steep’ before using ‘cg’.

My .mdp file is as follows:
minim.mdp - input into grompp to generate em.tpr for energy minimization
; define = -DFLEXIBLE
integrator = cg ; Algorithm (cg = conjugate gradient minimization)
emtol = 1000.0 ; Stop minimization when the maximum force < 1000.0 kJ/mol/nm
emstep = 0.01 ; Energy step size
nstcgsteep = 1000
nsteps = 5000 ; Maximum number of (minimization) steps to perform

Please assist me with how I can continue minimization after the steepest descent method with the cg method!

Thank you!

Since you have not specified the constraints keyword, it defaults to none, so the only constraints in the system are in water and require the use of define = -DFLEXIBLE, which you have commented out. If your constraints are failing, this means the system isn’t sufficiently minimized to proceed to CG minimization. Try using flexible water, then re-minimizing with whatever constraints are needed. If that fails, you probably can’t use CG, which is extremely sensitive. I never use it, honestly, due to the frequency of failure.