[Pw_forum] two program logic questions in TDDFPT

[kuilin lu]

Dear QE users and developers:
   I have two program logic questions in TDDFPT.  The questions are
about lr_lanczos.f90.

-----------------------
place 1:
/-----------------------------------------------------------------------------------------------------------\
place 1
273     IF(.not.ltammd) THEN
274     !
275     IF ( mod(LR_iteration,2)==0 ) THEN
276        CALL
lr_apply_liouvillian(evc1(1,1,1,1),evc1_new(1,1,1,1),sevc1_new(1,1,1,1),.false.)
277        CALL
lr_apply_liouvillian(evc1(1,1,1,2),evc1_new(1,1,1,2),sevc1_new(1,1,1,2),.true.)
278     ELSE
279        CALL
lr_apply_liouvillian(evc1(1,1,1,1),evc1_new(1,1,1,1),sevc1_new(1,1,1,1),.true.)
280        CALL
lr_apply_liouvillian(evc1(1,1,1,2),evc1_new(1,1,1,2),sevc1_new(1,1,1,2),.false.)
281     ENDIF
282     !
283     ELSE
284        CALL
lr_apply_liouvillian(evc1(1,1,1,1),evc1_new(1,1,1,1),sevc1_new(1,1,1,1),.true.)
285        CALL zcopy(size_evc,evc1(:,:,:,1),1,evc1(:,:,:,2),1)
!evc1(,1) = evc1(,2)
286        CALL
zcopy(size_evc,evc1_new(:,:,:,1),1,evc1_new(:,:,:,2),1) !evc1_new(,1)
= evc1_new(,2)
287        CALL
zcopy(size_evc,evc1_new(:,:,:,1),1,evc1_new(:,:,:,2),1) !evc1_new(,1)
= evc1_new(,2)
288
289     ENDIF
\-----------------------------------------------------------------------------------------------------------/
at line 287, it is
  CALL zcopy(size_evc,evc1_new(:,:,:,1),1,evc1_new(:,:,:,2),1)
!evc1_new(,1) = evc1_new(,2)
should here be the following?
  CALL zcopy(size_evc,sevc1_new(:,:,:,1),1,sevc1_new(:,:,:,2),1)
!sevc1_new(,1) = sevc1_new(,2)

  since it is a waste to copy evc1_new(:,:,:,1) two times and missing
setting value of sevc1_new(:,:,:,2)
\

-----------------------
place 2:
/-----------------------------------------------------------------------------------------------------------\
place 2
217     IF ( abs(beta)<1.0d-12 ) THEN
218        !
219        WRITE(stdout,'(5x,"lr_lanczos: Left and right Lanczos
vectors are orthogonal, this is a violation of oblique projection")')
220        !
221     ELSEIF ( beta<0.0d0 ) THEN
222        !
223        beta=sqrt(-beta)
224        gamma=-beta
225        !
226     ELSEIF ( beta>0.0d0 ) THEN
227        !
228        beta=sqrt(beta)
229        gamma=beta
230        !
231     ENDIF
\-----------------------------------------------------------------------------------------------------------
/
at line 217

  if beta is by accidently small, it just print a warning
“lr_lanczos: Left and right Lanczos vectors are orthogonal, this is a
violation of oblique projection”
then exit the IF structure. So variable gamma is actually not
computed, it will use the value of last step.  It might cause NaN
error later since beta and gamma are not same in this step and
different from lanczos algorithm
(http://www.cs.utk.edu/~dongarra/etemplates/node245.html) which is
mentioned in the code.

 Should here be the following?

!------
    IF ( abs(beta)<1.0d-12 ) THEN
       !
       WRITE(stdout,'(5x,"lr_lanczos: Left and right Lanczos vectors
are orthogonal, this is a violation of oblique projection")')
       !
    ENDIF
    IF ( beta<0.0d0 ) THEN
       !
       beta=sqrt(-beta)
       gamma=-beta
       !
    ELSEIF ( beta>0.0d0 ) THEN
       !
       beta=sqrt(beta)
       gamma=beta
       !
    ENDIF
!------
    I met this problem when I wrote my program based on Siesta (which
is a DFT software),  finding that  variable beta can be small, and get
NaN later.  My  program could get reasonable result after using my
changed logic. The test result is in accessory of this mail.  So I
doubt there is some problem here. If the original logic is right,  is
there some reason to do so? Thanks in advance.
 \

Best Wishes
Kuilin Lu
-- 
----------------------------------------------------------------------
Kuilin Lu, Ph.D student
College of Materials Science and Engineering
Hunan University
Changsha 410082
China
E-Mail: lukuilin at gmail.com
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