# Combine Spin-Flip TDDFT with Surface-Hopping

Dear all,

As is shown in the title, I tried to combine SF-TDDFT with FSSH to investigate S1->S0 process. For example, I tried this for ethylene.

Firstly, I performed a single point SF-TDDFT job to find singlet states. Below text is copied from output.

`````` Excited state   1: excitation energy (eV) =   -4.1688
Total energy for state  1:                   -78.52970653 au
<S**2>     :  0.0130
S(    2) --> S(    1) amplitude =  0.9900 alpha

Excited state   2: excitation energy (eV) =    0.9231
Total energy for state  2:                   -78.34258383 au
<S**2>     :  2.0177
S(    1) --> S(    1) amplitude =  0.7099 alpha
S(    2) --> S(    2) amplitude = -0.6948 alpha

Excited state   3: excitation energy (eV) =    3.9334
Total energy for state  3:                   -78.23195701 au
<S**2>     :  1.0132
D(    7) --> S(    1) amplitude =  0.9888

Excited state   4: excitation energy (eV) =    4.2523
Total energy for state  4:                   -78.22023620 au
<S**2>     :  0.0210
S(    1) --> S(    1) amplitude =  0.6990 alpha
S(    2) --> S(    2) amplitude =  0.7138 alpha

Excited state   5: excitation energy (eV) =    5.0981
Total energy for state  5:                   -78.18915523 au
<S**2>     :  1.0067
S(    2) --> V(    1) amplitude =  0.9903 alpha
``````

Hence, I hope to choose state 1 and 4 as states that to be considered in FSSH job. By applying “CIS_S2_Thresh 50”, it printed below error:

``````namdMethods: unable to find the active surface (2)
within the states used for this calculation
(below).
states = {1, 4}
``````

It seemed that correct singlet states were selected, however, the keyword “FSSH_INITIALSURFACE 2” ordered the program to use state 2 as initial surface, not the second state used for this calculation.

I thought “FSSH_INITIALSURFACE 4” may solve this error, however, due to crossing points may (and surely) exist on the relaxation path, the state number may change.

I wonder if there is any method can order the program to use “second in selected states” instead of “second in all states”.

Below is my FSSH input:

``````\$molecule
0 3
C         0.6690712511   -0.0268376644    0.0050315169
C        -0.6396456893   -0.1886733266   -0.0420316825
H         1.1181981658    0.8267926667    0.4872420937
H         1.3455134717   -0.7411370258   -0.4366902395
H        -1.0897934715   -1.0416754444   -0.5243097601
H        -1.3163849942    0.5252327237    0.4000552941
\$end

\$rem
JOBTYPE aimd
METHOD BHHLYP
BASIS 6-31G(d,p)
SCF_MAX_CYCLES 128
CIS_N_ROOTS 5
CIS_MAX_CYCLES 128
SPIN_FLIP True
CIS_S2_Thresh 50
SYMMETRY off
SYM_IGNORE true
PROJ_TRANSROT true
FSSH_LOWESTSURFACE 1
FSSH_NSURFACES 2
FSSH_INITIALSURFACE 2
AFSSH 0
CALC_NAC true
AIMD_STEPS 1000
TIME_STEP 21
AIMD_SHORT_TIME_STEP 3
AIMD_TIME_STEP_CONVERSION 1
AIMD_PRINT 1
AIMD_INIT_VELOC thermal
AIMD_TEMP 300 # K
AIMD_INTEGRATION vverlet
FOCK_EXTRAP_ORDER 6
FOCK_EXTRAP_POINTS 12
\$end
``````

Hi kalinite,

While it is possible in principle, functionality to do what you request is not currently implemented in Q-Chem.

-vale