1. index:: single: Program; Extf single: Extf

4.2.16. EXTF

This module calculates the contribution of an external force that is acting on the system. It applies the modification directly on the gradient and it needs to be called after the execution of ALASKA, in an optimization or molecular dynamics calculation. The keyword LINEAR applies a constant linear force between two atoms [81].

4.2.16.1. Input

4.2.16.1.1. General keywords

MODULE

Module of the force to apply, in nanonewton. If it’s negative, the force is applied in opposite direction. See the other keywords for what is the direction of positive and negative forces. Note that this is the module of the total force, so for example, in the case of a force pair between two atoms, the force applied on each atom will be a factor of \(\sqrt{2}\) smaller than this value.

LINEAR

This keyword is followed by two integer values, specifying the atom numbers (following the numbering of the geometry) between which a force is applied along the vector joining them. A positive force (see the MODULE keyword) means an attractive (compression) force, a negative force is a repulsive (extension) force.

BENDING

This keyword is followed by three integer values, specifying the atom numbers (following the numbering of the geometry) between which a force is applied to open or close their planar angle. A positive force (see the MODULE keyword) tends to close the angle, a negative force opens it.

TORSIONAL

This keyword is followed by four integer values, specifying the atom numbers (following the numbering of the geometry) between which a force is applied to open or close their dihedral angle. A positive force (see the MODULE keyword) tends to close positive dihedrals (i.e. towards less positive values), a negative force opens positive dihedrals (towards more positive values).

GAUSSIAN

This keyword modulates the applied force with a Gaussian time profile. It is followed by two real values, indicating the time at which the force is maximum (i.e. the value specified by MODULE) and a sigma value for the Gaussian decay.

4.2.16.1.2. Input examples

The following input example is a semiclassical molecular dynamics with tully surface hop, where a linear force of about 2.9 nN is applied between atom 1 and atom 2.

&Gateway
coord=$Project.xyz
basis=6-31G*
group=nosym

>> EXPORT MOLCAS_MAXITER=400
>> DOWHILE

&Seward

&rasscf
 nactel = 6 0 0
 inactive = 23
 ras2 = 6
 ciroot = 2 2 1
 prwf = 0.0
 mdrlxroot = 2

&alaska

&surfacehop
 tully
 decoherence = 0.1
 psub

&Extf
 LINEAR
 1 2
 MODULE
 -4.1

&Dynamix
 velver
 dt = 41.3
 velo = 1
 thermo = 0
>>> End Do

This example shows an excited state CASSCF MD simulation of a methaniminium cation using the Tully Surface Hop algorithm. In the simulation, the carbon and the nitrogen are pulled apart with a constant force of 1.5 nN (nanonewton) on each atom. Within the EXTF module the keyword LINEAR is used. Note EXTF needs to be called after the execution of ALASKA, inside the loop.

&GATEWAY
 COORD
 6
 Angstrom
 C  0.00031448  0.00000000  0.04334060
 N  0.00062994  0.00000000  1.32317716
 H  0.92882820  0.00000000 -0.49115611
 H -0.92846597  0.00000000 -0.49069213
 H -0.85725321  0.00000000  1.86103989
 H  0.85877656  0.00000000  1.86062860
 BASIS= 3-21G
 GROUP= nosym

>> EXPORT MOLCAS_MAXITER=1000
>> DOWHILE

&SEWARD

>> IF ( ITER = 1 )

&RASSCF
  LUMORB
 FileOrb= $Project.GssOrb
 Symmetry= 1
 Spin= 1
 nActEl= 2 0 0
 Inactive= 7
 RAS2= 2
 CIroot= 3 3 1

>> COPY $Project.JobIph $Project.JobOld

>> ENDIF

&RASSCF
 JOBIPH; CIRESTART
 Symmetry= 1
 Spin= 1
 nActEl= 2 0 0
 Inactive= 7
 RAS2= 2
 CIroot= 3 3 1
 MDRLXR= 2

>> COPY $Project.JobIph $Project.JobOld

&surfacehop
 TULLY
 SUBSTEP = 200
 DECOHERENCE = 0.1
 PSUB

&ALASKA

&extf
 LINEAR
 1 2
 MODULE
 -2.12132

&Dynamix
 VELVer
 DT= 10.0
 VELO= 3
 THER= 2
 TEMP=300

>> END DO