[gmx-users] difference of calculation time between the numbers of fep-lambdas values

[nao.morishita at takeda.com]

Dear Gromacs Users,

I would appreciate you to give me any comments or hints about the question below.
What makes the calculation time much longer when I calculate a solvation free energy using the mdp input1 compared to the mdp input2?

Input1
Free_energy           = yes
Couple-lambda0        = vdw-q
Couple-lambda1        = none
Couple-intramol       = no
Init-lambda-state     = 2
Fep-lambdas           = 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Calc-lambda-neighbors = 1
Input2
Free_energy           = yes
Couple-lambda0        = vdw-q
Couple-lambda1        = none
Couple-intramol       = no
Init-lambda-state     = 2
Fep-lambdas           = 0.0 0.1 0.2
Calc-lambda-neighbors = 1

It takes three times longer when I write down 36 lambda values compared to 3 lambda values even if there is no difference between mdp inputs other than the value of fep-lambdas and it is a normal free energy calculation not like Hamiltonian replica exchange calculation.

Thanks in advance and best regards,

Nao