Heat capacity is NaN

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GROMACS version: 2024.3-plumed_2.10.0
GROMACS modification: No

Hi everyone,

I am trying to compute the heat capacity of TIP4P water with gmx dos. I’ve performed EM, 2 ns NVT equilibration with the Nosé-Hoover thermostat, 2 ns NPT equilibration with the Berendsen barostat, and another NPT run with the Parrinello-Rahman barostat once the system was equilibrated. My simulation box contains 2211 water molecules. To compute the heat capacity, I ran a 200 ps simulation with these parameters:

; Run control
integrator = md
tinit = 0
dt = 0.001
nsteps = 200000 ; 200 ps
nstcomm = 100

; Output control
nstxout = 1
nstvout = 1
nstfout = 0
nstlog = 20000
nstenergy = 200
nstxout-compressed = 10000

Then I ran the command gmx dos -f prod_Cp.trr -s prod_Cp.tpr -dos dos.xvg -g dos.log. However, in the log file, I get:

DoS parameters
Delta = 0.179286
fluidicity = 0.25113
hard sphere packing fraction = 1.65778
hard sphere compressibility = -2.98674
ideal gas entropy = 0.0297878
hard sphere entropy = 0.0187333
sigma_HS = 0.288443 nm
DoS0 = 692.426
Dos2 = 6.40987e+14
DoSTot = 26513.1
Diffusion coefficient from VACF 3.45092 10^-5 cm^2/s
Diffusion coefficient from DoS 3.59111 10^-5 cm^2/s
Heat capacity -nan J/mol K

Any ideas why the heat capacity is NaN? Thanks!