AMBER Archive (2005)

Subject: AMBER: quadrupole in resp

From: riccardo nifosi (r.nifosi_at_nest.sns.it)
Date: Wed May 25 2005 - 05:44:20 CDT

Dear Amber members,

a collegue of mine pointed out what seems like a bug in how resp calculates
the quadrupole components. The resp subroutine QUADM reads:

C THIS SUBROUTINE CALCULATES THE COMPONENTS OF THE QUADROPOLE MOMENT

      DIMENSION C(3,maxq), q(maxq), QUAD(6)

      QXX=0.0d0
      QYY=0.0d0
      QZZ=0.0d0
      QXY=0.0d0
      QXZ=0.0d0
      QYZ=0.0d0

      DO 7 I=1,NATOM
         X2 = C(1,I)*C(1,I)
         Y2 = C(2,I)*C(2,I)
         Z2 = C(3,I)*C(3,I)
         RQ = X2 + Y2 + Z2
            QXX = QXX + Q(I)*( 3.d0* (C(1,I)*C(1,I)) - RQ )
            QYY = QYY + Q(I)*( 3.d0* (C(2,I)*C(2,I)) - RQ )
            QZZ = QZZ + Q(I)*( 3.d0* (C(3,I)*C(3,I)) - RQ )
            QXY = QXY + Q(I)*( 3.d0* (C(1,I)*C(2,I)) - RQ )
            QXZ = QXZ + Q(I)*( 3.d0* (C(1,I)*C(3,I)) - RQ )
            QYZ = QYZ + Q(I)*( 3.d0* (C(2,I)*C(3,I)) - RQ )
    7 CONTINUE

but the definition of quadrupole moment is
Q_ij = \int (3 x_i x_j - |x|^2 \delta_ij) \rho (x) d^3 x,
(\rho is the charge density and \delta_ij is the Kronecker delta)
where the -|x|^2 (-RQ in the subroutine) is present only in the diagonal
terms.
So I guess the term -RQ should'nt be there in QXY, QXZ and QYZ.

Thanks,
------------------------------------------------------------------
Riccardo Nifosė
NEST-INFM Scuola Normale Superiore
Piazza dei Cavalieri n. 7
I-56126 Pisa (IT)
                
Tel: +39 050509412 Fax: +39 050509417
Homepage: www.nest.sns.it/~nifosi/

-----------------------------------------------------------------------
The AMBER Mail Reflector
To post, send mail to amber_at_scripps.edu
To unsubscribe, send "unsubscribe amber" to majordomo_at_scripps.edu