# 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. At present time, just the LINEAR keyword is present, that applies a constant linear force between two atoms [68].

## 4.2.16.1. Input¶

### 4.2.16.1.1. General keywords¶

LINEAR
This keyword works by specifying 4 parameters, each one in its own line after the keyword itself. First parameter (Integer) is the first atom number following the numeration of the geometry. Second parameter (Integer) is the second atom number. Third parameter is the force (Float) in nanonewton applied along the vector between the two atoms. Fourth parameter is 0 or 1 (Bool), where 0 indicates a repulsive force, and 1 is for an attractive force.

### 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 2.9 nN is applied between atom 1 and atom 2.

&Gateway
coord=$Project.xyz basis=6-31G* group=nosym >> FOREACH A in (1..400) &Seward &rasscf nactel = 6 0 0 inactive = 23 ras2 = 6 ciroot = 2 2 1 prwf = 0.0 mdrlxroot = 2 &alaska &surfacehop tully decoherence = 0.1 psub &Extf LINEAR 1 2 2.9 0 &Dynamix velver dt = 41.3 velo = 1 thermo = 0 >>> End Do  This example shows an excited state CASSCF MD simulation of a methaniminium cation using the Tully Surface Hop algorithm. In the simulation, the carbon and the nitrogen are pulled apart with a constant force of 1.5 nN (nanonewton). Within the EXTF module the keyword LINEAR is used. Note EXTF needs to be called after the execution of ALASKA, inside the loop. The options are: 1: the atom number corresponding to the C atom, 2: the atom number corresponding to the N atom, 1.5: the force intensity, 0: to indicate a repulsive force. &GATEWAY COORD 6 Angstrom C 0.00031448 0.00000000 0.04334060 N 0.00062994 0.00000000 1.32317716 H 0.92882820 0.00000000 -0.49115611 H -0.92846597 0.00000000 -0.49069213 H -0.85725321 0.00000000 1.86103989 H 0.85877656 0.00000000 1.86062860 BASIS= 3-21G GROUP= nosym >> FOREACH ITER in (1 .. 1000) &SEWARD >> IF ($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

&extf
LINEAR
1
2
1.5
0

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

>> END DO