This is only required by a run with torsion forces. This is not used at present but is included for future development.
| Operands | none |
This instructs the
program to call the torsion (four-body) bond input and geometry setup
subroutine. This has two sub-directives, TORA
and ENDS.
| Operand types | I | 2A | 2A | 2A | 2A |
| Operand names | ILABEL | LABELI | LABELJ | LABELK | LABELL |
- ILABEL
- is the label of the torsion bond type. It must be the same integer as used in the TOHA sub-directive of the potential input routine to define the torsion force constant.
- LABELI, LABELJ, LABELK and LABELL
- are three species
labels (See BASI).
The torsion angle
is the angle between two planes. Each plane is defined by three species.
The two planes have two species in common. These species have the indices
i and j. The remaining two species have the indices k and l. There are
two possible types of torsion geometry; the first has one of the species
k and l bound to each of i and j, the second has both of the species
k and l bound to either i or j.
There
is a sign convention involved in the definition of .
is
defined such that the species k and l tend towards the same site, the
torsion angle will tend to 0 rather than to 
.
The form of the potential previously given (See TOHA)
is
The sign convention
for is
only important for N odd, for in this case 
will have the opposite sign to cos(0). The user should check very carefully
any literature values to ensure that the same sign convention is used.
The value of the sign s may be changed to correct any problems.
Mathematically the
torsion angle
is defined by the equations given below.
This directive is followed by up to 12 cards with the format
| A | A | F |
| KEYW1 | KEYW2 | CUTOFF |
KEYW1 may have one of the literal values RIJ RIK RJL RIL RJK RKL. KEYW2 may have one of the values MIN or MAX. The combination of these two keywords is used to define a cutoff (minimum or maximum) between two of the species whose labels are given on the TORA directive. By default the maximum cutoff is taken as 3 times the short range cutoff CUTPOT and the minimum is taken as zero. Normally for torsions three cutoffs need to be given. It is better to give all 6 maximum cutoffs as this avoids excessive searching by the input routine. The cutoffs are input in Angstroms. The program will then find all torsions in the unit cell which lie within these cutoffs and for which the species i, j, k, and l are the correct types given by LABELI, LABELJ, LABELK and LABELL. Thus about a given pair of centre species i and j there may be many contributions to the total torsion energy from many different pairs of outer species k and l. i will always be one of the unit cell basis atoms, but j, k and l may be translated by a lattice vector to another unit cell.
Some comment is
necessary about the symmetry the program assumes with respect to interchange
of pairs of particles. The torsion force energy expression is symmetric
with respect to interchange of the centre species i and j and with respect
to interchange of the outer species k and l. The program will take account
of this when it ennumerates the possible torsions. Thus if i and j are
the same species type, the program will look for an ij pair within the
cutoff. Then if k and l are the same species type, again the program will
look for a kl pair within the necessary cutoffs.
ENDS
This terminates the four-body geometry setup program and returns control to the main menu of directives.Printed output
If PRIN TORS 1 is set, the following is outputBONDS WITH 4 BODY INDEX index ARE CALCULATED BETWEEN LABELI LABELJ LABELK LABELL CUTOFFS ARE (IN ANGSTROMS) PAIR LABEL MINIMUM MAXIMUM
| LIST OF FOUR BODY INTERACTIONS | ||||||||||
| INDEX | I,J | K,L | RIJ | RIK | RJL | TORSION | ||||
| X | Y | Z | X | Y | Z | RJK | RIL | RKL | ANGLE | |
Coordinates output are orthonormal coordinates. RIJ, RIK, RJK, RJL, RIL, RKL are in Å. The torsion angle is output in degrees.
Error messages
ERROR - ONLY ONE TORS DIRECTIVE IS PERMITTED
Only one TORS directive is permitted before the START PLUT directive.INVALID DIRECTIVE IN FOUR BODY FORCE INPUT
A sub-directive has been found which is neither TORA nor ENDS.ERROR - A FOUR BODY BOND CUTOFF HAS BEEN GIVEN WITH AN INVALID KEYWORD KEY USE KEYWORDS MIN OR MAX
ERROR - AN INVALID DIRECTIVE HAS BEEN READ IN. INPUT LINE IS
In reading in the cutoffs, an invalid keyword was found.TOO MANY FOUR BODY INTERACTIONS READ IN
The memory requirements exceed that available. On the CRAY, rerun the job with a larger value for the MFL= parameter on the JOB card.Examples
The first of these is for a carbonate ion in calcite. TOHA
1 5.0 1 2
.........
TORS
TORA 1 C CORE O CORE O CORE O CORE
RIJ MAX 1.5
RIK MAX 1.5
RIL MAX 1.5
RJK MAX 3.0
RJL MAX 3.0
RKL MAX 3.0
ENDS
TOHA
1 5.0 1 2
TOHA 2 5.0 -1 3
TOHA 3 5.0 1 2
TOHA 4 5.0 -1 3
.........
TORS
TORA 1 CO CORE OC CORE OH CORE CH CORE
RIJ MAX 1.3
RIK MAX 1.4
RIL MAX 1.6
TORA 2 CH CORE CH CORE HC CORE HC CORE
RIJ MAX 1.6
RIK MAX 1.2
RJL MAX 1.2
TORA 3 CO CORE OH CORE HO CORE OC CORE
RIJ MAX 1.6
RJK MAX 1.6
RIL MAX 1.6
TORA 4 CH CORE CH CORE CO CORE HC CORE
RIJ MAX 1.6
RIK MAX 1.6
RJL MAX 1.6
ENDS