Hi all,

I calculated MECP using a penalty-constrained optimization algorithm between S0 and S1 states. I used the SF-TDDFT method for this.

Here is my input:

$molecule

0 3

coordinates

$end

$rem

BASIS = cc-pVDZ

METHOD = wB97X-D

JOB_TYPE = optimization

MECP_OPT = true

mecp_methods = penalty_function

MAX_CIS_CYCLES = 400

SCF_CONVERGENCE = 8

MAX_SCF_CYCLES = 400

GEOM_OPT_MAX_CYCLES = 300

THRESH = 14

sts_mom = true

UNRESTRICTED = true

SPIN_FLIP = true

mecp_state1 = [0,1]

mecp_state2 = [0,2]

cis_s2_thresh = 120

CIS_N_ROOTS = 4

SYMMETRY_IGNORE = true

SYMMETRY = false

SOLVENT_METHOD = PCM

$end

$solvent

DIELECTRIC 32.613000

OPTICALDIELECTRIC 1.765709

$end

and In output I got:

at the last optimization cycle:

Excited state 1: excitation energy (eV) = 0.4876

Total energy for state 1: -899.00611584 au

<S**2> : 0.1121

S( 2) → S( 1) amplitude = 0.9704 alpha

S( 2) → V( 2) amplitude = 0.1743 alpha

Excited state 2: excitation energy (eV) = 0.5069

Total energy for state 2: -899.00540911 au

<S**2> : 1.0957

S( 1) → S( 1) amplitude = 0.9881 alpha

Excited state 3: excitation energy (eV) = 0.8563

Total energy for state 3: -898.99256568 au

<S**2> : 0.9926

D( 67) → S( 2) amplitude = 0.1521

S( 2) → S( 2) amplitude = 0.9652 alpha

Excited state 4: excitation energy (eV) = 2.7267

Total energy for state 4: -898.92383098 au

<S**2> : 1.0458

D( 65) → S( 1) amplitude = -0.2151

D( 66) → S( 1) amplitude = -0.8870

D( 67) → S( 1) amplitude = -0.3176

D( 69) → S( 1) amplitude = 0.1715

and

– Optimization of the CI objective function (Cycle # 2) –

CI_Sigma (Lagrange multiplier) = 14.000

CI_Alpha (stability parameter) = 0.020 a.u.

Lower state energy (state 1) = -899.0061158375 a.u.

Upper state energy (state 2) = -899.0054091106 a.u.

Energy gap = 0.000707 a.u. (0.019 eV)

Value of F (objective function) = -899.00542478 a.u.

Value of sigma*G (penalty) = +0.00033769 a.u.

```
Convergence criteria value thresh cnvgd?
Component of F^x parallel to G^x: -0.00000606 0.00030000 YES
Component of F^x perpendicular to G^x: +0.00017731 0.00030000 YES
Change in the value of F: -0.00000013 0.00000100 YES
Energy gap: +0.00070673 0.00100000 YES
```

I want to ask here that In this it is considering E.S-1 and E.S-2 as S0 and S1 states respectively but E.S-2 S2 value is 1.09. I have given CIS_S2_THRESH = 120 which is why it is considered it singlet state but normally can we say that the calculated results are correct? This is the energy gap between S0 and S1 not between S0 and T1?