Error in TD-DFT (setman/rpa_dia.C): Unable to diagonalize A-B

Questions
#1

Currently I am using IP-tuned wB97XD3 with and without PCM on aggregates of perylene derivatives. For one of the systems the calculations crashes after the TDA calculation with the error:

 Direct TDDFT calculation will be performed
 Exchange:     0.1957 Hartree-Fock + 1.0000 wB97X-D3 + LR-HF
 Correlation:  1.0000 wB97X-D3
 Using SG-2 standard quadrature grid
 Dispersion:   Grimme D3
 ---------------------------------------------------
 Iter    Rts Conv    Rts Left    Ttl Dev     Max Dev
 ---------------------------------------------------

 Q-Chem fatal error occurred in module setman/rpa_dia.C, line 59:

 Unable to diagonalize A-B

The input options read as follows:

$rem
BASIS def2-SVP
METHOD wB97X-D3
CIS_N_ROOTS      8
CIS_SINGLETS     true
CIS_TRIPLETS     false
RPA              true
OMEGA 140
SOLVENT_METHOD = PCM
state_analysis    true
GUI = 2
MAX_SCF_CYCLES = 1000
MEM_TOTAL = 7000
$end

$pcm
Theory      CPCM
$end

$solvent
Dielectric 3
OpticalDielectric 3
$end

Is it possible to use a different algorithm for the digonalization of A-B matrices?
Thank you
Sara

#2

This could probably do with a better error message, rather than the default one that is thrown by BLAS, but when you see that the problem is almost always a triplet instability in the reference determinant. This manifests as a negative eigenvalue in the orbital Hessian (A-B), so the calculation is fundamentally ill-posed. The solution is to use the Tamm-Dancoff approximation (RPA=0, which is the default), as the B matrix is not needed and A is positive definite. Alternatively you can try a different density functional, cross fingers that you get a stable ground-state solution. Hartree-Fock theory is generally more susceptible to triplet instabilities (especially at stretched bond lengths) and functionals with larger fractions of exact exchange tend to inherent this sensitivity.

#3

Thank you for the quick answer and sorry for the late reply. It was indeed a triplet instability. It just took me quite some more investigations to believe it, since I did not at all expect a triplet ground state for this systems. As it happens, I messed up my structure input.

Thanks again and sorry for occupying your time with my rookie mistakes.

#4

No need to apologize, that error is rather cryptic if you’re not thinking about instabilities.