Author Topic: Negative excitation energies in ADC(2)  (Read 320 times)

dcav

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Negative excitation energies in ADC(2)
« on: January 18, 2019, 07:51:58 pm »
Dear all,
    I am trying to obtain a relaxed scan of furan and some other molecules in the S1 state, using sos-ri-adc(2)/def2-tzvp, and increasing C-O distance. Several of the initial geometries appear to approach conical intersections well documented in the literature. However, a couple of these geometries have a negative S1 excitation energy. How should I interpret this? Is this calculation still reliable?

   Here is a fragment of the control file for the ricc2 calculation:

$denconv     0.10000000E-06
$freeze
 implicit core=    5 virt=    0
$cbas    file=auxbasis
$ricc2
  adc(2)
  geoopt model=adc(2)    state=(a 1)
  sos   cos= 1.30000
  maxiter=60
$excitations
  irrep=a  multiplicity=  1  nexc=  4  npre=  5  nstart=  6
$closed shells
 a       1-18                                   ( 2 )


   Also, some of the output of ricc2.

 +==================================================================+
 | sym | multi | state |          CCS excitation energies           |
 |     |       |       +--------------------------------------------+
 |     |       |       |   Hartree    |     eV       |     cm-1     |
 +==================================================================+
 | a   |   1   |   1   |    0.0077019 |      0.20958 |     1690.370 |
 | a   |   1   |   2   |    0.1500643 |      4.08346 |    32935.306 |
 | a   |   1   |   3   |    0.1510398 |      4.11000 |    33149.398 |
 | a   |   1   |   4   |    0.2295911 |      6.24749 |    50389.415 |
 | a   |   1   |   5   |    0.2469002 |      6.71850 |    54188.332 |
 +==================================================================+
 

+================================================================================+
 | sym | multi | state |          ADC(2) excitation energies    |  %t1   |  %t2   |
 |     |       |       +----------------------------------------+--------+--------+
 |     |       |       |   Hartree    |    eV      |    cm-1    |    %   |    %   |
 +================================================================================+
 | a   |   1   |   1   |   -0.0086017 |   -0.23406 |  -1887.848 |  89.97 |  10.03 |
 | a   |   1   |   2   |    0.0606178 |    1.64949 |  13304.069 |  82.25 |  17.75 |
 | a   |   1   |   3   |    0.1137216 |    3.09452 |  24959.007 |  90.70 |   9.30 |
 | a   |   1   |   4   |    0.1630437 |    4.43665 |  35783.961 |  87.32 |  12.68 |
 +================================================================================+

   Thanks in advance for any help,

   Daniel.

Arnim

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Re: Negative excitation energies in ADC(2)
« Reply #1 on: January 21, 2019, 10:12:28 am »
Hello Daniel,

it could be that the ground state of these geometries can not be decribed with a single-reference wave function. Then you would have to apply multi-reference methods to sudy them.
However, having not looked at the systems in detail, this is only a guess.

Cheers,

Arnim