# Vertical excitation energies and Noneq PCM

Hello All,

In a test calculation for vertical excitation energy calculation for singlets and triplets for water molecule in the presence of toluene (just an example of a solvent) I find that the lowest two singlet excitation energies are close to zero if I specify CIS_N_ROOTS as 2 or more. If I specify CIS_N_ROOTS as 1 then the lowest excitation energy is correctly reported to be 6.61eV. Am I missing something in the input. Please see the inputs below, the only option that changes between the inputs is CIS_N_ROOTS. I have the Gen option below because I am testing this input for some other molecule with heavy atoms with additional ECP input (not included here).

\$molecule
0 1
H 0.000000 -0.115747 1.133769
H 0.000000 1.109931 -0.113383
O 0.000000 0.005817 -0.020386
\$end

\$rem
EXCHANGE cam-b3lyp
BASIS Gen
SCF_CONVERGENCE 10
XC_GRID 000099000590
THRESH 16
MAX_SCF_CYCLES 600
SCF_ALGORITHM diis
MEM_STATIC 3000
MEM_TOTAL 20000
solvent_method pcm
MAX_CIS_CYCLES 80
CIS_N_ROOTS 4
CIS_CONVERGENCE 8
CIS_SINGLETS true
CIS_TRIPLETS false
CIS_DYNAMIC_MEM true
SET_ITER 300
RPA false
\$end

\$pcm
Nonequilibrium
THEORY IEFPCM
vdwScale 1.1
\$end

\$solvent
DIELECTRIC 2.3741
OpticalDielectric 2.2407
\$end

\$basis
O 0
6-31g(d,p)

H 0
6-31g(d,p)

\$end

Thanks for any pointers!
Jayashree

An example output in the excitation energy section is given below:

``````     TDDFT/TDA Excitation Energies
``````

Excited state 1: excitation energy (eV) = 0.0000
Total energy for state 1: -76.35448433 au
Multiplicity: Singlet
Trans. Mom.: -0.0000 X -0.0000 Y -0.0000 Z
Strength : 0.0000000000

Excited state 2: excitation energy (eV) = 0.0002
Total energy for state 2: -76.35447871 au
Multiplicity: Singlet
Trans. Mom.: 0.0000 X 0.0000 Y 0.0000 Z
Strength : 0.0000000000

Excited state 3: excitation energy (eV) = 6.6113
Total energy for state 3: -76.11152264 au
Multiplicity: Singlet
Trans. Mom.: -0.1868 X 0.0000 Y -0.0000 Z
Strength : 0.0056504511
D( 5) → V( 1) amplitude = 0.9981

I’m not sure what the issue is but it appears to be fixed in the current version of the code. Using your input file and the Q-Chem trunk version, I get

`````` ---------------------------------------------------
TDDFT/TDA Excitation Energies
---------------------------------------------------
#### No ptSS-corrected energies - see below ####

Excited state   1: excitation energy (eV) =    6.6185
Total energy for state  1:                   -76.11126024 au
Multiplicity: Singlet
Trans. Mom.:  0.1845 X   0.0000 Y  -0.0000 Z
Strength   :     0.0055203415
D(    5) --> V(    1) amplitude =  0.9981

Excited state   2: excitation energy (eV) =    8.2712
Total energy for state  2:                   -76.05052274 au
Multiplicity: Singlet
Trans. Mom.:  0.0460 X   0.0000 Y  -0.0000 Z
Strength   :     0.0004292063
D(    5) --> V(    2) amplitude =  0.9975

``````

That message about “No ptSS-corrected energies” is new (they appear in a subsequent block), was added during some updates for Q-Chem v. 6.1 so perhaps this bug has been fixed. Please contact Q-Chem support by email and let them know what version of Q-Chem you are using, and they can test and see if there is a workaround that doesn’t simply require you to update your version of Q-Chem.

Thanks. I am using version 5.3. I will contact the QChem support and update here.

I have emailed QChem support twice but haven’t heard from them, it’s been almost a week. Is anyone else facing this problem with version 5.3 and has a workaround?

Thanks,
Jayashree

I notice that the above problem goes away if I don’t use IEF-PCM and SS(V)PE. Does anyone else have issues using these two solvent theories?

In that case I strongly suggest using C-PCM instead of IEF-PCM. (C-PCM is the default.) There is very little practical difference between these two models; see: https://doi.org/10.1002/wcms.1519

Hello Jayashree,

Sorry for the delay in response. The issue originates from numerical instability in the Gram-Schmidt algorithm. We fixed this issue in 5.4.1. Unfortunately, there is no workaround for 5.3. If you have 5.4 license, you can upgrade to 5.4.2 complimentarily. Please let us know if you have additional questions.

Kuan-Yu

Thanks John and Kuan-Yu. Apologies for the late reply from my side. Thanks for the suggestion, I will use C-PCM instead for the moment. I do have to upgrade to version 6 if possible soon!