Input Parameter Values

FAQ on input parameters:

• If I set the keyword RESTART WAVEFUNCTION COORDINATES, would I have to write the &SYSTEM and &ATOM sections again?
• Could anybody tell me how to choose the energy cutoff in &SYSTEM section?
• I have a problem with visualising unoccupied orbitals. Either I get only occupied orbitals, or, if I add one empty state when optimizing wavefunction the program never reaches convergence.
• Is there any way to force CPMD to dump DENSITY https every N steps of a molecular dynamics run?
• How do I calculate a Band structure with CPMD?
• I want to collide fast atoms against surfaces. Why are the initial velocities given using the VELOCITIES keyword not considered in the calculation?
• I want to run CPMD with basis sets equivalent to Gaussian 6-31+G(d) and 6-311+G(2d,p). How do i set up the &BASIS section?
• How do I add support for a new functional?

Q: If I set the keyword RESTART WAVEFUNCTION COORDINATES, would I have to write the &SYSTEM and &ATOMS sections again?

A: Yes, you have to include the &SYSTEM and &ATOMS sections even if you are restarting. If you write RESTART COORDINATES, the coordinates in the RESTART http override the ones in the input. RESTART WAVEFUNCTION alone does not select the coordinates in the RESTART http, but does use those in the &ATOMS section.

Q: Could anybody tell me how to choose the energy cutoff in &SYSTEM section?

A: The best way to choose the cutoff for CPMD calculations is by running first a series of tests. Select a test system and a representative quantity (bond length, reaction energy, etc.), perform a series of calculations with increasing cutoff, pick the lowest cutoff with satisfactory results.

It's always a good idea to make checks at some critical points of the calculations by increasing the cutoff. See also section 9.1.

Q: I have a problem with visualising unoccupied orbitals. When I use RHOOUT BANDS or CUBEFILE ORBITALS after the wavefunction optimization I get only occupied orbitals. If I add one empty state when optimizing wavefunction the program never reaches convergence.

A: The most efficient way to calculate unoccupied orbitals is to first optimize the occupied orbitals and then restart the calculation using the run option

 KOHN-SHAM ENERGIES
  n

where n ist the number of unoccupied orbitals. This will diagonalize the Kohn-Sham Potential (defined by the occupied orbitals alone).

To test if everything goes fine, you can check the total energy printed at the beginning of this job, it should be exactly the one at the end of the optimization. In addition, if you don't change the default convergence criteria, the number of converged Kohn-Sham states should be equal to the number of occupied states in the first step.

Q: Is there any way to force CPMD to dump DENSITY https every N steps of molecular dynamics run instead (or except) of the end of the job?

A: Short of modifying the source code, you could set the parameter RESTFILE to a large number and than have CPMD write a restart http every N steps via the STORE keyword. Now you rename each restart in turn from RESTART.# to RESTART and do a single step calculation using the RESTART keyword without the LATEST modifier which will write the DENSITY http (or run a PROPERTIES job using CUBEFILE DENSITY to get the cube http directly).

Q: How do I calculate a Band structure with CPMD? To calculate a band structure with CPMD, You first calculate the correct density for your system with a Monkhorst-Pack Mesh.

A: Then you use: OPTIMIZE WAVEFUNCTIONS with MAXSTEP 1 (no self-consistency) and RESTART DENSITY.

In the section KPOINTS, you should use for instance a bcc:

   KPOINTS BANDS
        51   0   0   0      0   0   1             Gamma to H
        51   0   0   1      0  .5  .5             H to N
        51   0  .5  .5     .5  .5  .5             N to P
        51  .5  .5  .5      0   0   0             P to Gamma
        51   0   0   0     .5  .5   0             Gamma to N
        51   0   0   1     .5  .5  .5             H to P
        0    0 0 0  0 0 0

You say that you want 51 points from (0,0,0) and (0,0,1) and so on. The last line with many zeros is to stop.

If the memory of your computer is not enough, you can add in the line KPOINTS the option BLOCK=50 that means you want to have only 50 kpoints in memory. This options worked some time ago.

Q: I've been recently trying to use the VELOCITIES keyword in a molecular dynamics run. I want to collide fast atoms against surfaces. Despite the code seems to read the input velocities properly, when the run starts the initial velocities are always the same (apparently coming from a thermal distribution), no matter what is the velocity you specify for the incoming atom. I'm not using QUENCH IONS, so I don't understand why the input initial velocities are not considered in the calculation.

A: There is no straightforward way in CPMD to achieve what you want. I suggest to follow this procedure:

1) Run a single step of MD with the following set up

MOLECULAR DYNAMICS
MAXSTEP
 1
RESTART WAVEFUNCTION COORDINATES
TEMPERATURE
  300   <- or whatever your surface should be

This generates RESTART and GEOMETRY https. Now edit the GEOMETRY http to change the velocities of the particles according to your experiment. Now restart the MD with the options

MOLECULAR DYNAMICS
MAXSTEP
 1000
RESTART WAVEFUNCTION COORDINATES VELOCITIES GEOFILE
QUENCH ELECTRONS

The effect of this is: IONIC coordinates and velocities are read from GEOMETRY, ELECTRON wavefunctions and velocities are read from RESTART, ELECTRON velocities are set to zero.

Q: I want to run CPMD with basis sets equivalent to Gaussian 6-31+G(d) and 6-311+G(2d,p). How do I set up the &BASIS section?

A: You should be able to construct inputs from the description in this manual (see section 11.5.3).

Please note, that the basis set generated from the &BASIS section is used in CPMD for two purposes:

1. Analyzing orbitals:
   We usually use the atomic pseudo-wavefunctions to analyze the orbitals from CPMD. The 6-31G type Gaussian basis sets are for all electron calculations. Don't expect very good results when analyzing wavefunctions from a pseudopotential calculation.
2. Generating orbitals for an initial guess:
   By default we use a Slater minimal basis. In most cases the effort to produce a better initial guess using ``better'' wavefunctions does not pay off.

Q: How do I add support for a new functional?

A: Have a look at the http dftin.F, where you can see how CPMD reads the &DFT section and then follow the flow of the defined variables through the https, e.g. functionals.F, gcener.F, lsd_func.F, and so on.