ROMP help

Caspar Fall (c.j.fall@exeter.ac.uk)
January 2001

Warning: This information is provided primarily for users at the University of Exeter!

ROMP

pre-Relaxation and Oscillations using the Musgrave-Pople potential
(Musgrave, M. J. P. and Pople, J. A., Proc. R. Soc. London, Ser. A 268 (1962) 474).

Description:

romp uses exclusively the Musgrave-Pople (MP) potential for pre-relaxation. It knows the correct MP parameters for various semiconductors. romp should be able to read a dat file compatible with the supercell or cluster code, or an output file from either version. It supports comments in the dat file (i.e. lines that start with %). It should automatically generate the same bonds as the supercell code and fully supports include and exclude commands.rompwill read the bonds from an input or output file, but can also re-generate them using the -map option. It will generate all bonds smaller than a given distance, specified using the -b option.

romp can also calculate localised vibrational modes, isotopic shifts, and phonon band structures, using the Musgrave-Pople potential and any AIMPRO-produced second derivatives. If you are calculating vibrational modes from an output file, romp will substitute the second derivatives calculated numerically in the output file into the dynamical matrix (unless you use to -nodd option). If you want immobile atoms (such as hydrogens fixed at the surface of a cluster), set their type to X or x, or alternatively specify that they should be kept fixed using the -f option.

Warning: romp is still buggy and is provides as is. If it produces rubbish output or refuses to relax a cluster, it's probably that there are too many or too few bonds for some atoms. It is therefore recommended to examine the output and examine the number of bonds generated for each atom. If romp hangs when generating the bonds, try reducing the specified bond length.

Usage:

romp [-options] dat.file / out.file

Options:

-h    : show this help
-s    : use only springs in calculation
-sa   : use only springs and angles in calculation
-last : use last atomic positions in file (default)
-first: use first atomic positions in file
-m # : relax only first # atoms
-map : regenerate bond map from data file
-b # : set automatic bond generation length to #
-dat : write new dat file (& no extra atoms in xyz)
-nor : do not perform structural relaxation
-v    : calculate phonons
-i # #: calc isotopic shifts for atoms # #
-f # #: fix atom types # # for vibrations
-nodd : do not use double derivs from output
-n # : use about # kpts for phonon band structure
-g    : use only gamma point (default)
-k    : read kpts for phonons from dat.bandst
-wz   : use wurtzite kpts for phonon band structure (A-L-M-G-A-H-K-G)
-fcc : use fcc      kpts for phonon band structure (L-G-X-U-K-G)
-lin #1 #2 #3 : line of kpts along #1 b1 + #2 b2 + #3 b3
-rep #1 #2 #3 : output #1 * #2 * #3 unit cells
-ot   : output top   vibrational mode of first kpt
-o # : output #-th vibrational mode of first kpt
-t # : output top # vibrational modes of first kpt
-pbs #1 #2: calc b3-projected band structure along #1 b1 + #2 b2
-nx # : in pbs, use # kpts along #1 b1 + #2 b2
debugging options (not for everyday use):
-d # : set the debug level (default=1)
-ov   : override warnings

Output files:

xyz.relax.dat.file        : relaxed position of atoms
phon.dat.file             : vibrational modes of atoms
gnu.dat.file              : gnuplot file for plotting phon.dat.file
vib.*.dat.file            : xyz file with vibrational mode *, suitable for
                            visualisation with minimol
loc.*.dat.file            : localisation information for vib mode *

Optional input file:

dat.bandst: should contain the k-points for calculating phonons (the first line should have the number of points, the following ones have the k-points coordinates in relative units, i.e. multiples of the reciprocal lattice vectors).

Examples:

romp dat.file             : relax supercell dat.file
romp -s dat.file          : relax supercell dat.file using springs
romp -m 4 dat.file        : relax first 4 atoms
romp -nor dat.file        : don't relax, just show energy and bonds
romp -v dat.file          : relax, then calculate phonons at Gamma
romp -v -o 9 dat.file     : calc phonons, and output 9th vib mode
romp -v -k dat.file       : calc phonons using k-points in dat.bandst

romp -nor -v -lin 0 0 1 dat.file : calc phonons along b3, to edge of BZ
romp -nor -v -fcc -n 90 dat.file : ~90 k-pts along a L-G-X-U-K-G circuit

Sample output:

Here is a sample annotated output for calculating the vibrational modes of a SiC cluster with a H interstitial (atom No. 177). We impose no relaxation of the atomic coordinates, and fix the position of the 3rd atomic species, which are the hydrogens decorating the surface of the cluster. The resulting animation is shown at the top of the page. The output is produced using:

romp -v -nor -f 3 c-sic:h0_ab2_cl176.out

############################# R O M P ################################
# pre-Relaxation and Oscillations using the Musgrave-Pople potential #
#                   (c.j.fall@exeter.ac.uk, 2000)                    #
######################################################################
Reading a cluster output file...
Read last atomic positions in file...
total atoms : 177
moving atoms : 177
types : Si C H H
setting up neighbour maps and padding...
step 1 2 3 4
derivatives computed for atoms: 1 2 6 7 8 177
reading double derivatives for 6 atoms
nlines / nhalf lines: 3 0
padding: 0 0
bonds:
from    1 to    2   3   4   5       <---bonds to these atoms are considered
from    2 to    1   6   7   8 177
from    3 to    1 13 18 30
from    4 to    1 12 15 26
from    5 to    1 16 20 24
from    6 to    2 11 17 28
from    7 to    2   9 14 23
from    8 to    2 10 19 31
from    9 to    7 12 22 52
from   10 to    8 16 27 55
from   11 to    6 20 27 50
from   12 to    4   9 25 43
from   13 to    3 14 25 47
[...more output...]
from 175 to   98
from 176 to   98
from 177 to    2
mx bond: 0.
bonds : 236
supercell size greater than 3: ok

bond:   length:   spring:
SiC      3.5300    3.6599           <--- MP stretch parameters
SiH      3.0000    1.0000
C Si     3.5300    3.6599
C H      2.2000    2.0000
C H      2.2000    2.0000
H Si     3.0000    1.0000
H C      2.2000    2.0000
H C      2.2000    2.0000
type:   bend str-bend   str-str bend-bend
Si    0.4749    0.1807   -0.0513    0.0315 <-- MP bending parameters
C     0.2916    0.0852    0.7591    0.0270 <--
H     0.0812    0.0288    0.0202    0.0100 <--
H     0.0812    0.0288    0.0202    0.0100 <--

egy == Total energy ==      (eV)
egy    stretch       bend stre-bend    str-str bend-bend      total
egy   1.949388   1.450241   0.003483   0.177013 -0.083892   3.496233
                                           MP total energy -----^
Calculating vibrational modes...
** Using double derivatives from file! **
reciprocal unit vectors:
     0.00006     0.00000     0.00000
     0.00000     0.00006     0.00000
     0.00000     0.00000     0.00006
q-vec:        qx       qy       qz       qnorm     set-up   diagon   remain
   1 from   1: 0.00000 0.00000 0.00000 0.00000     9.64    21.18     0.00

Output localisation: loc.c-sic:h0_ab2_cl176.out

The loc.c-sic:h0_ab2_cl176.out file contains the calculated vibrational modes (atom 177 is the interstitial H atom):

# vibrational modes of c-sic:h0_ab2_cl176.out
#mode   freq \ at:    1        2        6        7        8      177   total
[...lines of output...]
525   829.353 0.00433 0.00069 0.00288 0.00288 0.00288 0.00106 0.01473
526   831.584 0.00000 0.00000 0.00295 0.00295 0.00295 0.00000 0.00885
527   832.925 0.00230 0.00218 0.00066 0.00065 0.00072 0.10520 0.11170
528   832.925 0.00230 0.00218 0.00069 0.00070 0.00063 0.10520 0.11170
529   863.396 0.00199 0.01214 0.00505 0.00018 0.00366 0.95841 0.98143
530   863.396 0.00199 0.01214 0.00087 0.00575 0.00227 0.95841 0.98143
531 3044.485 0.00000 0.00778 0.00000 0.00000 0.00000 0.99221 1.00000
         ^                                                       /        ^
         |                 % localisation of LVM on atom 177 ---/         |
        LVM frequency      % localisation of LVM on atoms 1+2+6+7+8+177 --/

Sample output (2):

Here is a phonon band structure for bulk cubic SiC calculated usingromp. It is a one-line operation once the data filec-sic.dathas been set up:

romp -v -nor -fcc -n 100 -map c-sic.dat

Caspar Fall              email: C.J.Fall@exeter.ac.uk
University of Exeter     phone: +44 (1392) 264198
Exeter EX4 4QL, UK       fax  : +44 (1392) 264111

24th January 2001