# AMBER: quadrupole in resp

From: riccardo nifosi <r.nifosi.nest.sns.it>
Date: Wed, 25 May 2005 12:44:20 +0200

Dear Amber members,

a collegue of mine pointed out what seems like a bug in how resp calculates

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/

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Received on Wed May 25 2005 - 11:53:03 PDT
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