subroutine refsys(i2,i3,i4,e1,e2,e3,fail) implicit real*8 (a-h,o-z) include 'DIMENSIONS' c this subroutine calculates unity vectors of a local reference system c defined by atoms (i2), (i3), and (i4). the x axis is the axis from c atom (i3) to atom (i2), and the xy plane is the plane defined by atoms c (i2), (i3), and (i4). z axis is directed according to the sign of the c vector product (i3)-(i2) and (i3)-(i4). sets fail to .true. if atoms c (i2) and (i3) or (i3) and (i4) coincide or atoms (i2), (i3), and (i4) c form a linear fragment. returns vectors e1, e2, and e3. logical fail double precision e1(3),e2(3),e3(3) double precision u(3),z(3) include 'COMMON.IOUNITS' include "COMMON.CHAIN" data coinc /1.0d-13/,align /1.0d-13/ fail=.false. s1=0.0d0 s2=0.0d0 do 1 i=1,3 zi=c(i,i2)-c(i,i3) ui=c(i,i4)-c(i,i3) s1=s1+zi*zi s2=s2+ui*ui z(i)=zi 1 u(i)=ui s1=sqrt(s1) s2=sqrt(s2) if (s1.gt.coinc) goto 2 write (iout,1000) i2,i3,i1 fail=.true. return 2 if (s2.gt.coinc) goto 4 write(iout,1000) i3,i4,i1 fail=.true. return 4 s1=1.0/s1 s2=1.0/s2 v1=z(2)*u(3)-z(3)*u(2) v2=z(3)*u(1)-z(1)*u(3) v3=z(1)*u(2)-z(2)*u(1) anorm=sqrt(v1*v1+v2*v2+v3*v3) if (anorm.gt.align) goto 6 write (iout,1010) i2,i3,i4,i1 fail=.true. return 6 anorm=1.0/anorm e3(1)=v1*anorm e3(2)=v2*anorm e3(3)=v3*anorm e1(1)=z(1)*s1 e1(2)=z(2)*s1 e1(3)=z(3)*s1 e2(1)=e1(3)*e3(2)-e1(2)*e3(3) e2(2)=e1(1)*e3(3)-e1(3)*e3(1) e2(3)=e1(2)*e3(1)-e1(1)*e3(2) 1000 format (/1x,' * * * error - atoms',i4,' and',i4,' coincide.') 1010 format (/1x,' * * * error - atoms',2(i4,2h, ),i4,' form a linear') return end