SeqQuest input - Molecular dynamics


SeqQuest
Home
User
Guides
Input
Manual

Table of Contents

  1. Overview
  2. Input options
  3. Details and defaults
  4. MD methods and thermostats
  5. Diagnostic tools
  6. Troubleshooting

Overview

This page gives a description of the input section that controls and modifies how a molecular dynamics run is done. The MD input section is optional; all the user need do is add a "do dynamics" instruction in the command options and the code will automatically launch a molecular dynamics run, with an NVE simulation at room temperature.

The default units for the dynamics section are Ry for energies, bohr for distances, Kelvin for temperature, and femtoseconds for time.

Current constraints, important version notes:


Input options

The molecular dynamics input section can appear anywhere in the "run phase" or "geometry relaxation" section of the input file, begins with the "dynamics data" keyword and ends with the "end dynamics" keyword.

The following are the common input keywords in this section. All the keywords are optional, and can appear in any order within this section. The keywords must be left-justified, but all data input is free-format.

dynamics input - begin MD input section
...
timestep - time step (femtoseconds) for dynamics
dt_md
md_steps - number of (thermalized stage) MD steps
n_step_md
md_method - molecular dynamics method
mdtype ( NVE | BERENDSEN | HOOVER | TSCALE )
md_temperature - molecular dynamics temperature (Kelvin)
temp_md
md_parameter - parameter defining MD method (see below) (2.61)
param_md
equilibration steps - number of equilibration steps
n_step_eq
eq_method - equilibration method (2.61)
eqtype ( NVE | BERENDSEN | HOOVER | TSCALE )
eq_temperature - equilibration target temperature (Kelvin)
temp_eq
eq_parameter - parameter defining eq method (see below) (2.61)
param_eq
...
end dynamics input - end of MD input section

Details and defaults

The dynamics section enables the SeqQuest program to run molecular dynamics on the Born-Oppenheimer potential energy surface. The units of temperature are Kelvin, the unit of time is fs, and the length of simulation is in number of timesteps.

Temperature. The default temperature for the dynamics is 300 K, room temperature, unless the user specifies an input md_temperature.

Initialization. For initialization (if there is no MD restart file), velocities are randomly assigned from a Boltzmann distribution oriented in random directions. The initial velocities are consistent with a temperature twice the simulation temperature. Note: this implicitly assumes the atomic positions are at, or near, equilibrium positions, such that half the energy will go into potential modes and thereby leading to an equilibrated temperature half that of the temperature defined by the initial velocities. If the atoms are away from initial positions, the internal energy they possess will result in a higher equilibrated temperature than the target simulation temperature.

Equilibration. The equilibration phase takes the initialized position and velocities and runs a number of equilibration steps specified by the user with the eq_steps keyword. The default number of equilibration steps is 0, but 100-1000 steps are typical values. The default equilibration scheme is TSCALE. The user can select a any valid thermostat with the eq_method keyword, and input a parameter to tune that thermostat with eq_parameter keyword.

Dynamics. After the system has been equilibrated, the MD phase begins. The user may specify the number of time steps to be run with the md_steps keyword. The default number of time steps is 1000000, to effectively run until the user kills the job, or the job times out. The user may specify the type of molecular dynamics to be run (see below) with the "md_method" keyword. The default dynamics method is NVE.

Restart. To continue a dynamics run from a previous set of positions and velocities, keep the last "lcao.vxyz" file from the previous run and place it in the running directory of the SeqQuest job. This MD job will then continue from the last point in the previous run (with the latest coordinates, velocities, and thermostat conditions).

The vxyz file begins with a line specifying the number of atoms in the molecule, followed by a title line. These lines are followed by one line for each atom in the unit cell containing a symbol for the atom (ignored by SeqQuest), and six floating point fields (input in free format) containing the x, y, and z coordinates, and the vx, vy, and vz velocities. At the end of the vxyz file is a set of additional data necessary to consistently continue the thermostat. The vxyz file is a variant of the XMol XYZ Format, except that instead of records for only the coordinates, the velocity fields are added. If the vxyz file is present, the positions of the atoms in the SeqQuest input file will be overwritten by the values in the vxyz file.


MD methods and thermostats

The update scheme used in the molecular dynamics is leapfrog verlet. The default thermostat for dynamics is NVE, and the default thermostat for the equilibration is TSCALE. The available thermostats are:

Diagnostic tools

There are a few built-in diagnostic tools in the MD output:

Troubleshooting

The dynamics module will print some warning messages if parameters are input that appear suspect:
WARNING: Quest MD -- Temperature high
indicates that the input temperature is outside of the range that SeqQuest believes will work.
WARNING: Quest MD -- Large time step entered
indicates that the time step entered is too large for SeqQuest to accurately integrate Newton's equations
WARNING: Quest MD -- Small time step entered
indicates that the time step entered is very small, and that too many time steps will be required to obtain meaningful statistics.
WARNING: likely uninitialized mass
suggests that an atom file may have been input without field describing the atomic mass (note: the default mass is 1.0, and Hydrogen in your system may be incorrectly flagged as not being initialized).
Other problems that may occur:
With NVE, a larger energy drift is observed
Reduce the size of the time step in the MD.
Slow equilibration for NVE (or Hoover, Berendsen):
Use an aggressive temperature scaling method to pre-equilibrate (e.g., a 100fs Berendsen, 20fs Hoover, TSCALE, etc.).
Poor equipartition (uneven temperature distribution among atoms)
Return to Top or Input Manual or User Guides or SeqQuest Home
Send questions and comments to: Rick Muller at rmuller@sandia.gov
Last updated: January 17, 2014