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fort.71 file — MD energy, temperature, and pressure report

The fort.71 file contains time-series thermodynamic information from a ReaxFF molecular dynamics (MD) simulation.

It is the primary file for monitoring: - Energy conservation - Temperature control - Pressure behavior - Convergence during MD or MD-energy minimization

Purpose of the fort.71 file

fort.71 is used to:

  • Track potential, kinetic, and total energy
  • Monitor MD temperature statistics
  • Inspect pressure evolution
  • Diagnose MD stability and equilibration
  • Determine convergence in MD-energy minimization runs

Output frequency

The output frequency is controlled by the control-file keyword:

iout1

A line is written to fort.71 every iout1 MD iterations.

In Example 3.5, iout1 = 5, so output appears every 5 iterations.

File structure

  • No header comments
  • One line per output step
  • Fixed column order with 16 columns

Example 3.5: fort.71 output file

Iter. Nmol Epot Ekin Etot T(K) Eaver(b) Eaver(tot) Taver Tmax Pres sdev1 sdev2 Tset Tstep RMSG

5   1 1 -663.7 0.8 -662.9 38.3 -663.8 -663.8 41.3 46.0 0.0 0.1 0.1 50.0 0.5 22.2
10  1 1 -663.9 0.9 -663.0 42.4 -663.8 -663.8 42.2 44.5 0.0 0.0 0.0 50.0 0.5 28.7
15  1 1 -663.8 0.9 -662.9 43.9 -663.8 -663.8 42.5 43.9 0.0 0.0 0.0 50.0 0.5 18.9
20  1 1 -663.9 1.0 -662.9 49.9 -663.9 -663.8 48.6 49.9 0.0 0.0 0.0 50.0 0.5 7.4
25  1 1 -663.7 0.8 -662.9 37.1 -663.7 -663.8 40.5 45.4 0.0 0.1 0.0 50.0 0.5 25.0
30  1 1 -663.8 0.5 -663.3 24.6 -663.8 -663.8 21.5 25.6 0.0 0.1 0.0 50.0 0.5 31.1
35  1 1 -663.8 0.5 -663.3 25.0 -663.8 -663.8 25.4 27.0 0.0 0.0 0.0 50.0 0.5 15.0
40  1 1 -663.7 0.5 -663.2 25.3 -663.7 -663.8 24.3 25.4 0.0 0.0 0.0 50.0 0.5 13.1
45  1 1 -663.7 0.6 -663.2 26.9 -663.7 -663.8 24.9 26.9 0.0 0.0 0.0 50.0 0.5 30.0

Column definitions

Column Description
Iter MD iteration number
Nmol Number of molecules (using cutof2 bond criterion)
Epot Total potential energy
Ekin Total kinetic energy
Etot Total energy (Epot + Ekin)
T(K) Instantaneous MD temperature
Eaver(b) Block-averaged potential energy over last iout1 steps
Eaver(tot) Average potential energy over entire run
Taver Average temperature over last iout1 steps
Tmax Maximum temperature in last iout1 steps
Pres MD pressure (MPa, intermolecular contribution only)
sdev1 Std. deviation of Epot over last iout1 steps
sdev2 Std. deviation of average Epot over entire run
Tset Target (set) temperature
Tstep MD timestep (fs)
RMSG Root-mean-square force

Notes on pressure

The pressure reported in fort.71: - Is based on intermolecular interactions only - Is not reliable for evaluating pressure in condensed phases - Should be interpreted qualitatively rather than quantitatively

Termination behavior

In MD-energy minimization runs (imetho = 2):

  • ReaxFF terminates when:
  • RMSG < endmd (from control file), or
  • Maximum MD iterations are reached

Practical usage

  • Primary file for MD diagnostics
  • Often plotted as:
  • Epot vs iteration
  • Temperature vs iteration
  • Parsed routinely in automated MD analysis pipelines

Summary

  • fort.71 reports MD thermodynamics and convergence
  • Output frequency controlled by iout1
  • Central to equilibration and stability analysis
  • Essential companion to xmolout for MD workflows