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SUBROUTINE GOVER(NI,NJ,XI,XJ,R,SG)
************************************************************************
* *
* GOVER CALCULATES THE OVERLAP INTEGRALS USING A GAUSSIAN EXPANSION *
* STO-6G BY R.F. STEWART, J. CHEM. PHYS., 52 431-438, 1970 *
* *
* ON INPUT NI = ATOMIC NUMBER OF FIRST ATOM *
* NJ = ATOMIC NUMBER OF SECOND ATOM *
* R = INTERATOMIC DISTANCE IN ANGSTROMS *
* ON EXIT S = 9X9 ARRAY OF OVERLAPS, IN ORDER S,PX,PY, *
* PZ *
* *
************************************************************************
IMPLICIT DOUBLE PRECISION (A-H,O-Z)
INCLUDE 'SIZES'
COMMON /NATYPE/ NZTYPE(107), MTYPE(30),LTYPE
COMMON /TEMP/ C(60,6), Z(60,6)
COMMON /NATORB/ NATORB(107)
DIMENSION S(6,6), XI(3),XJ(3), SG(9,9)
SAVE NGAUSS
DATA NGAUSS/6/
C
C FIND START AND END OF GAUSSIAN
C
IFA=NZTYPE(NI)*4-3
IF(C(IFA+1,1).NE.0.D0)THEN
ILA=IFA+3
ELSE
ILA=IFA
ENDIF
IFB=NZTYPE(NJ)*4-3
IF(C(IFB+1,1).NE.0.D0)THEN
ILB=IFB+3
ELSE
ILB=IFB
ENDIF
C
C CONVERT R INTO AU
C
R=R/0.529167D0
R = R**2
KA=0
DO 80 I=IFA,ILA
KA=KA+1
NAT=KA-1
KB=0
DO 80 J=IFB,ILB
KB=KB+1
NBT=KB-1
C
C DECIDE IS IT AN S-S, S-P, P-S, OR P-P OVERLAP
C
IF(NAT.GT.0.AND.NBT.GT.0) THEN
C P-P
IS=4
TOMB=(XI(NAT)-XJ(NAT))*(XI(NBT)
1-XJ(NBT))*3.5711928576D0
ELSEIF(NAT.GT.0) THEN
C P-S
IS=3
TOMB=(XI(NAT)-XJ(NAT))*1.88976D0
ELSEIF(NBT.GT.0) THEN
C S-P
IS=2
TOMB=(XI(NBT)-XJ(NBT))*1.88976D0
ELSE
C S-S
IS=1
ENDIF
DO 60 K=1,NGAUSS
DO 60 L=1,NGAUSS
S(K,L)=0.0D0
AMB=Z(I,K)+Z(J,L)
APB=Z(I,K)*Z(J,L)
ADB=APB/AMB
C
C CHECK OF OVERLAP IS NON-ZERO BEFORE STARTING
C
IF((ADB*R).LT.90.D0) THEN
ABN=1.0D0
GO TO(50,10,20,30),IS
10 ABN=2.D0*TOMB*Z(I,K)*SQRT(Z(J,L))/AMB
GO TO 50
20 ABN=-2.D0*TOMB*Z(J,L)*SQRT(Z(I,K))/AMB
GO TO 50
30 ABN=-ADB*TOMB
IF(NAT.EQ.NBT) ABN=ABN+0.5D0
40 ABN=4.0D0*ABN*SQRT(APB)/AMB
50 S(K,L)=SQRT((2.D0*SQRT(APB)/AMB)**3)*EXP(-ADB*R)*
. ABN
ENDIF
60 CONTINUE
SG(KA,KB)=0.0D0
DO 70 K=1,NGAUSS
DO 70 L=1,NGAUSS
70 SG(KA,KB)=SG(KA,KB)+S(K,L)*C(I,K)*C(J,L)
80 CONTINUE
RETURN
END
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