questions about TDDFPT2

[Matt W]

Hi Hanning,

I've not had a chance to look at your particular calculation. Such a small 
basis set, and small box could lead to strange results in any case. More 
generally, there is no reason, even if the transition is a pure single 
determinant HOMO to LUMO, for the energy to be that of the HOMO_LUMO gap. 
There are still additional terms in the TDDFT kernel that will shift the 
transition energy.
For GGAs the shifts can be in either direction, there is no general rule.
We have checked small molecule transition energies vs gamess-us and were 
pretty happy with the comparison.

HTH,

Matt


On Sunday, February 5, 2017 at 8:50:49 PM UTC, NUCP2K wrote:
>
> Dear CP2K Developers and Users,
>
>   I just tried out the latest implementation of time-dependent density 
> functional perturbation theory (TDDFPT) in CP2K, and found some serious 
> inconsistencies on the calculated excitation energy. 
>
>   I tested the TDDFPT2 on an isolated water (H2O) molecule using the 
> attached input file. Interestingly, the calculated excitation energy of 
> 10.075 eV  is much much greater than the HOMO-LUMO gap of 9.137 eV, even 
> though the transition is dominated (>99.9%) by the electron transfer from 
> HOMO to LUMO.  
>
>   It seems to me that the diagonal terms of the TDDFPT response matrix are 
> not appropriately assigned, leading to the apparent inconsistency. Perhaps, 
> the subroutine "tddfpt_init_by_energy_diff" in qs_tddfpt2_methods.F should 
> be fixed to yield correct orbital energy gap. 
>
>   Please correct me if my understanding is wrong.
>
>   Thanks.
>
> Hanning
>
>
>          &FORCE_EVAL
>   METHOD QS
>   &PROPERTIES
>     &TDDFPT
>      NSTATES      1
>      MAX_ITER    100
>      MAX_KV 10
>      CONVERGENCE [eV] 1.0E-3
>      &DIPOLE_MOMENTS
>       DIPOLE_FORM LENGTH
>      &END DIPOLE_MOMENTS
>      &MGRID
>       COMMENSURATE
>       CUTOFF 50
>      &END MGRID
>     &END TDDFPT
>   &END PROPERTIES
>   &DFT
>     BASIS_SET_FILE_NAME BASIS_SET
>     POTENTIAL_FILE_NAME POTENTIAL
>     &MGRID
>       COMMENSURATE
>       CUTOFF 50
>     &END MGRID
>     &POISSON
>      PERIODIC NONE
>      POISSON WAVELET
>     &END POISSON
>     &SCF
>       SCF_GUESS ATOMIC
>     &END SCF
>     &XC
>       &XC_FUNCTIONAL PBE
>       &END XC_FUNCTIONAL
>       &XC_GRID
>         XC_DERIV SPLINE2_SMOOTH
>       &END XC_GRID
>     &END XC
>   &END DFT
>   &SUBSYS
>     &CELL
>       ABC 6.0 6.0 6.0
>       PERIODIC NONE
>     &END CELL
>     &COORD
>    O     3.000000     3.000000     3.000000    H2O1
>    H     3.000000     3.000000     4.000000    H2O1
>    H     3.942809     3.000000     2.666667    H2O1
>     &END COORD
>     &KIND H
>       BASIS_SET SZV-GTH
>       POTENTIAL GTH-PADE-q1
>     &END KIND
>     &KIND O
>       BASIS_SET SZV-GTH
>       POTENTIAL GTH-PADE-q6
>     &END KIND
>     &TOPOLOGY
>     &END TOPOLOGY
>   &END SUBSYS
> &END FORCE_EVAL
> &GLOBAL
>   PROJECT WATER
>   PRINT_LEVEL MEDIUM
>   RUN_TYPE ENERGY
> &END GLOBAL
>
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