[Pw_forum] virtual states in TDDFT (turbo_lanczos.x) calculations

[Giuseppe Mattioli]

Dear all
I've performed a TDDFT calculation of the absorption spectrum of a molecule, and I am playing around 
with the new (QE 5.0) "stage 2" calculations, which should indicate the contribution of virtual 
states to the absorption coefficients, as explained in

my_Espresso/5.0/TDDFPT/Examples/CH4-BOND

The "stage 2" calculation has been pointed on the first absorption band found by turbo_spectrum.x; my 
molecule has 98 occupied states, and 10 unoccupied have been calculated by pw.x (nband=108) and I 
suppose to be quite satisfied (for all I know of this molecule) with these lines

      98   1     0.28374685E+04   -0.82400471E+04   2.33588
      98   2     0.27984762E+04   -0.82731097E+04   2.34525
      98   3    -0.52066456E-05    0.13245374E-04   0.00000
      98   4    -0.93634066E-05    0.23337916E-04   0.00000
      98   5     0.52748267E-05   -0.14618050E-04   0.00000
      98   6    -0.78431969E+01    0.20124113E+02  -0.00570
      98   7    -0.78379694E+01    0.20102365E+02  -0.00570
      98   8    -0.10283141E-04    0.18715947E-04   0.00000
      98   9    -0.29200741E-04    0.36618310E-04   0.00000
      98  10     0.38666959E-05   -0.12074233E-04   0.00000

which seem to indicate that the onset of the absorption spectum can be attributed to single-particle 
transitions involving the highest occupied state and the first and second virtual states (I daresay 
HOMO connected with LUMO and LUMO+1...).

But what about these lines?

      92   1    -0.67396365E+03    0.17328962E+04  -0.49124
      92   2    -0.68404352E+03    0.17306015E+04  -0.49059
      92   3     0.94419011E-05   -0.60010033E-04   0.00000
      92   4    -0.18274042E-04    0.26892556E-04   0.00000
      92   5    -0.51970803E-04    0.11972278E-03   0.00000
      92   6    -0.38956134E+01    0.48423966E+01  -0.00137
      92   7    -0.38946693E+01    0.48428589E+01  -0.00137
      92   8    -0.20625070E-01   -0.59183925E-04   0.00000
      92   9    -0.17290402E-02   -0.34538054E-04   0.00000
      92  10     0.59116529E-05   -0.18839515E-04   0.00000

That is, what do negative (and not so small) coefficients means?

Thank you in advance

Giuseppe

-- 
********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et 
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM)          
   Tel + 39 06 90672836 - Fax +39 06 90672316    
   E-mail: <giuseppe.mattioli at ism.cnr.it>