subroutine gen_rand_conf_mchain(nstart0,*)
C Generate random conformation or chain cut and regrowth.
      implicit none
      include 'DIMENSIONS'
      include 'COMMON.CHAIN'
      include 'COMMON.LOCAL'
      include 'COMMON.VAR'
      include 'COMMON.INTERACT'
      include 'COMMON.IOUNITS'
      include 'COMMON.MCM'
      include 'COMMON.GEO'
      include 'COMMON.CONTROL'
      logical overlap_mchain,back,fail
      integer nstart,nstart0
      integer i,ii,iii,j,k,it,it1,it2,nit,niter,nsi,maxsi,maxnit
      integer igr,iequi,ichain,nnres
      double precision aux
      double precision gen_theta,gen_phi,dist,ran_number,scalar
c      write (iout,*)  'gen_rand_conf_mchain: maxgen=',maxgen
      nstart=nstart0
      maxsi=100
c      write (iout,*) 'Gen_Rand_conf_mchain: nstart=',nstart,
c     &   " nchain_group",nchain_group

      DO IGR=1,NCHAIN_GROUP

      DO IEQUI=1,NEQUIV(IGR)

      ichain=iequiv(iequi,igr)

      i=chain_border1(1,ichain)+nstart-1
      if (nstart.eq.1) then
        do j=1,3
          c(j,i)=ran_number(-15.0d0,15.0d0)
          dc(j,i-1)=c(j,i)
        enddo
      endif
      if (nstart.le.2) then

      do j=1,3
        dc_norm(j,i)=ran_number(-1.0d0,1.0d0)
      enddo
      aux=dsqrt(scalar(dc_norm(1,i),dc_norm(1,i)))
      do j=1,3
        dc_norm(j,i)=dc_norm(j,i)/aux
      enddo
      if (itype(i).eq.ntyp1) then
        do j=1,3
          dc(j,i)=1.9d0*dc_norm(j,i)
        enddo
      else 
        do j=1,3
          dc(j,i)=3.8d0*dc_norm(j,i)
        enddo
      endif
      do j=1,3
        c(j,i+1)=c(j,i)+dc(j,i)
      enddo
      endif
c      if (nstart.lt.5) then
      if (nstart.le.2) then
      it1=iabs(itype(i+2))
      phi(i+3)=gen_phi(i+3,iabs(itype(i+1)),iabs(itype(i+2)))
c       write(iout,*)'phi(4)=',rad2deg*phi(4)
      theta(i+2)=gen_theta(iabs(itype(i+2)),pi,phi(i+3))
      if (it1.ne.10) then
        nsi=0
        fail=.true.
        do while (fail.and.nsi.le.maxsi)
          call gen_side(it1,theta(i+2),alph(i+1),omeg(i+1),fail)
          nsi=nsi+1
        enddo
        if (nsi.gt.maxsi) then
          write (iout,'(a,i7,a,i7,a,i7,a,i7)') 
     &    'Problem in SC rotamer generation, residue',ii,
     &    ' chain',ichain,' chain group',igr,'. Increase MAXSI.'
          return1
        endif
      endif ! it1.ne.10
      call orig_frame_chain(i)
      else
c      call orig_frame_chain(i-1)
c      write (iout,*) "calling refsys",i
      call refsys(i,i-1,i-2,prod(1,1,i-1),
     &  prod(1,2,i-1),prod(1,3,i-1),fail)
c      write (iout,*) "after refsys",i
#ifdef DEBUG
      write (iout,*) "dc_norm(:",i-1,") and prod"
      do j=1,3
        write (iout,*) j,dc_norm(j,i-1),(prod(j,k,i-1),k=1,3)
      enddo
#endif
      endif

      ENDDO

      nstart=nstart+1
      ii=nstart

      maxnit=5000

      nit=0
      niter=0
      back=.false.
      nnres=chain_border1(2,iequiv(1,igr))-
     &  chain_border1(1,iequiv(1,igr))+1
#ifdef DEBUG
      write (iout,*) "chain group",igr," chains",
     &    (iequiv(j,igr),j=1,nequiv(igr))
      write (iout,*) "ii",ii," nnres",nnres
#endif
      do while (ii.le. nnres .and. niter.lt.maxgen)

        ichain=iequiv(1,igr)
        i=ii-1+chain_border1(1,ichain)
#ifdef DEBUG
        write (iout,*) "ii",ii," nnres",nnres," ichain",ichain," i",i,
     &    "niter",niter," back",back," nstart",nstart
#endif
        if (ii.lt.nstart) then
c          if(iprint.gt.1) then
          write (iout,'(/80(1h*)/2a/80(1h*))') 
     &          'Generation procedure went down to ',
     &          'chain beginning. Cannot continue...'
          write (*,'(/80(1h*)/2a/80(1h*))') 
     &          'Generation procedure went down to ',
     &          'chain beginning. Cannot continue...'
c          endif
          return1
        endif
        it1=iabs(itype(i-1))
        it2=iabs(itype(i-2))
        it=iabs(itype(i))
        if (it.eq.ntyp1 .and. it1.eq.ntyp1) then
          vbld(i)=ran_number(3.8d0,10.0d0)
          vbld_inv(i)=1.0d0/vbld(i)
        endif
#ifdef DEBUG
        write (iout,*) 'Gen_Rand_Conf: i=',i,' it=',it,' it1=',it1,
     &    ' it2=',it2,' nit=',nit,' niter=',niter,' maxgen=',maxgen
#endif
        phi(i)=gen_phi(i,it2,it1)
        phi(i+1)=gen_phi(i+1,it1,it)
#ifdef DEBUG
        write (iout,*) "phi",i,phi(i)," phi",i+1,phi(i+1)
#endif
        do iequi=2,nequiv(igr)
          iii=ii+chain_border1(1,iequiv(iequi,igr))
          phi(iii)=phi(i+1)
        enddo
#ifdef CHUJ
        if (back) then
          phi(i)=gen_phi(i+1,it2,it1)
#ifdef DEBUG
          write(iout,*) 'phi(',i,')=',phi(i)," type",it1,it2,it
#endif
          theta(i-1)=gen_theta(it2,phi(i-1),phi(i))
          if (theta(i-1).gt.2.68780478d0) theta(i-1)=2.68780478d0
          do iequi=2,nequiv(igr)
            iii=ii-1+chain_border1(1,iequiv(iequi,igr))
            phi(iii)=phi(i+1)
            theta(iii-1)=theta(i-1)
          enddo
          if (it2.ne.10 .and. it2.ne.ntyp1) then
            nsi=0
            fail=.true.
            do while (fail.and.nsi.le.maxsi)
              call gen_side(it2,theta(i-1),alph(i-2),omeg(i-2),fail)
              nsi=nsi+1
            enddo
            do iequi=2,nequiv(igr)
              iii=ii-3+chain_border1(1,iequiv(iequi,igr))
              alph(iii)=alph(i-2)
              omeg(iii)=omeg(i-2)
            enddo
#ifdef DEBUG
            write (iout,*) "alpha",alph(i)," omeg",omeg(i)," fail",fail
#endif
            if (nsi.gt.maxsi) then
              write (iout,'(a,i7,a,i7,a,i7,a,i7)') 
     &        'Problem in SC rotamer generation, residue',ii,
     &        ' chain',ichain,' chain group',igr,'. Increase MAXSI.'
              return1
            endif

          endif
c          call locate_next_res(i-1)
        endif
#endif
        if (it1.ne.ntyp1) then
          theta(i)=gen_theta(it1,phi(i),phi(i+1))
          if (theta(i).gt.2.68780478d0) theta(i)=2.68780478d0
        else
          theta(i)=ran_number(1.326d0,2.548d0)
        endif
#ifdef DEBUG
        write (iout,*) "ii",ii," i",i," it1",it1,
     &   " theta",theta(i)," phi",
     &    phi(i),phi(i+1)
#endif
        if (it1.ne.10 .and. it1.ne.ntyp1) then 
        nsi=0
        fail=.true.
        do while (fail.and.nsi.le.maxsi)
c          theta(i)=gen_theta(it1,phi(i),phi(i+1))
          call gen_side(it1,theta(i),alph(i-1),omeg(i-1),fail)
          nsi=nsi+1
        enddo
#ifdef DEBUG
        write (iout,*) "alpha",alph(i)," omeg",omeg(i)," fail",fail
#endif
c        write (iout,*) "After forward SC generation:",nsi,maxsi
        if (nsi.gt.maxsi) then
          write (iout,'(a,i7,a,i7,a,i7,a,i7)') 
     &    'Problem in SC rotamer generation, residue',ii,
     &    ' chain',ichain,' chain group',igr,'. Increase MAXSI.'
          return1
        endif

        endif

        do iequi=2,nequiv(igr)
          iii=ii-1+chain_border1(1,iequiv(iequi,igr))
          theta(iii)=theta(i)
          phi(iii)=phi(i)
          alph(iii-1)=alph(i-1)
          omeg(iii-1)=omeg(i-1)
        enddo

        DO IEQUI=1,NEQUIV(IGR)

        ichain=iequiv(iequi,igr)
        i=ii-1+chain_border1(1,ichain)
#ifdef CHUJ
        if (back) call locate_next_res(i-1)
#endif
        call locate_next_res(i)
#ifdef DEBUG
        write (iout,*) "i",i," ii",ii," ichain",ichain
        write (iout,*) theta(i)*rad2deg,phi(i)*rad2deg,
     &    alph(i-1)*rad2deg,omeg(i-1)*rad2deg
        write (iout,*) (c(j,i),j=1,3),(c(j,i+nres-1),j=1,3)
#endif
        if (overlap_mchain(i-1,ii-1,ichain,igr)) then
#ifdef DEBUG
          write (iout,*) "***********overlap",i-1," nit",nit
#endif
          if (nit.lt.maxnit) then
            back=.true.
            nit=nit+1
            exit
          else
#ifdef DEBUG
            write (iout,*) "***********overlap maxnit exceeded",nit
#endif
            nit=0
            if (ii.gt.3) then
              back=.true.
              ii=ii-1
c              write (iout,*) "ii",ii
              exit
            else
              write (iout,'(a,i7,a,i7,a,i7,a,i7)') 
     &        'Cannot generate non-overlaping conformation, residue',ii,
     &        ' chain',ichain,' chain group',igr,'. Increase MAXNIT.'
              return1
            endif
          endif
        else
c          write (iout,*) "No overlap",i-1
          back=.false.
c          nit=0 
c          i=i+1
        endif

        ENDDO

        if (.not.back) then
#ifdef DEBUG
          write (iout,*) "++++++++++Successful generation",igr,ichain,ii
#endif
          ii=ii+1
          nit=0
        endif
        back=.false.
c        write (iout,*) "ii",ii
        niter=niter+1

      enddo
      if (niter.ge.maxgen) then
        write (iout,'(a,2i7,a,i7,a,i7,a,i7)') 
     & 'Too many trials in conformation generation',niter,maxgen,
     & ' chain group',igr,' chain',ichain,' residue',ii
        return1
      endif

      ENDDO

      do i=2,nres
        if (itype(i).eq.ntyp1) then
          do j=1,3
            dc(j,i-1)=c(j,i)-c(j,i-1)
          enddo
        endif
      enddo

      return
      end
c-------------------------------------------------------------------------
      logical function overlap_mchain(i,ii,ichain,igr)
      implicit none
      include 'DIMENSIONS'
      include 'COMMON.IOUNITS'
      include 'COMMON.CHAIN'
      include 'COMMON.INTERACT'
      include 'COMMON.FFIELD'
      double precision redfac /0.5D0/,redfacp /0.8d0/,redfacscp /0.8d0/
      integer i,ii,ichain,igr,j,jj,jchain,jgr,k,iti,itj,iteli,itelj
      double precision rcomp
      double precision dist
      logical lprn /.false./
      overlap_mchain=.false.
      iti=iabs(itype(i))
      if (iti.gt.ntyp) return
C Check for SC-SC overlaps.
cd    print *,'nnt=',nnt,' nct=',nct
#ifdef DEBUG
      write (iout,*) "overlap_mchain i",i," ii",ii," ichain",ichain,
     &   " igr",igr," iti",iti
#endif
      do j=nnt,nct
        if (itype(j).eq.ntyp1) cycle
        jchain=ireschain(j)
c        write (iout,*) "overlap_mchain j",j," jj",jj," jchain",jchain,
c     &    " itype",itype(j)
        jgr=mapchain(jchain)
        jj=j-chain_border1(1,jchain)+1
        itj=iabs(itype(j))
c        write (iout,*) "jgr",jgr
        if(igr.eq.jgr.and.jj.gt.ii .or. ichain.eq.jchain .and. j.gt.i-1
     &    .or. jgr.gt.igr .and. jj.gt.nran_start) cycle
        if (j.lt.i-1 .or. ipot.ne.4) then
          rcomp=sigmaii(iti,itj)
        else 
          rcomp=sigma(iti,itj)
        endif
cd      print *,'j=',j
        if (dist(nres+i,nres+j).lt.redfac*rcomp) then
          overlap_mchain=.true.
          if (lprn) write(iout,*)'overlap_mchain, SC-SC: i=',i,' j=',j,
     &     ' ichain',ichain,' jchain',jchain,
     &     ' dist=',dist(nres+i,nres+j),' rcomp=',
     &     rcomp*redfac
        return
        endif
      enddo
#ifdef CHUJ
C Check for overlaps between the added peptide group and the preceding
C SCs.
      iteli=itel(i)
      if (iteli.gt.0) then
      do j=1,3
        c(j,maxres2+1)=0.5D0*(c(j,i)+c(j,i+1))
      enddo
      do j=nnt,nct
        itj=iabs(itype(j))
        if (itj.eq.ntyp1) cycle
        jchain=ireschain(j)
        jgr=mapchain(jchain)
        jj=j-chain_border1(1,jchain)+1
        if(igr.eq.jgr.and.jj.gt.ii .or. ichain.eq.jchain .and. j.gt.i-2
     &    .or. jgr.gt.igr .and. jj.gt.nran_start) cycle
cd      print *,'overlap_mchain, p-Sc: i=',i,' j=',j,
cd   &         ' dist=',dist(nres+j,maxres2+1)
        if (dist(j,maxres2+1).lt.4.0D0*redfacscp) then
          if (lprn) write (iout,*) 'overlap_mchain, p-SC: i=',i,' j=',j,
     &     ' ichain',ichain,' jchain',jchain,
     &     ' dist=',dist(nres+j,maxres2+1),' rcomp=',
     &    4.0d0*redfac 
          overlap_mchain=.true.
          return
        endif
      enddo
      endif
C Check for p-p overlaps
#endif
      iteli=itel(i)
      if (iteli.eq.0) return
      do j=1,3
        c(j,maxres2+1)=0.5D0*(c(j,i)+c(j,i+1))
      enddo
c      do j=nnt,i-2
      do j=nnt,nct
        itelj=itel(j)
        if (itelj.eq.0) cycle
        do k=1,3
          c(k,maxres2+2)=0.5D0*(c(k,j)+c(k,j+1))
        enddo
        jchain=ireschain(j)
        jgr=mapchain(jchain)
c        if(iteli.ne.0.and.itelj.ne.0)then
c        write (iout,*) i,j,dist(maxres2+1,maxres2+2),rpp(iteli,itelj)
        jj=j-chain_border1(1,jchain)+1
c        write (iout,*) "jgr",jgr
        if(igr.eq.jgr.and.jj.gt.ii .or. ichain.eq.jchain .and. j.gt.i-2
     &    .or. jgr.gt.igr .and. jj.gt.nran_start) cycle
#ifdef DEBUG
          write (iout,*)'overlap_mchain, p-p: i=',i,' j=',j,' igr',igr,
     &   ' jgr',jgr,' ichain',ichain,' jchain',jchain,
     &   ' dist=',dist(maxres2+1,maxres2+2)
#endif
        if (dist(maxres2+1,maxres2+2).lt.rpp(iteli,itelj)*redfacp) then
          if (lprn) write (iout,*) 'overlap_mchain, p-p: i=',i,' j=',j,
     &     ' ichain',ichain,' jchain',jchain,
     &     ' dist=',dist(maxres2+1,maxres2+2),' rcomp=',
     &    rpp(iteli,itelj)*redfacp
          overlap_mchain=.true.
          return
        endif
c        endif
      enddo
      return
      end