C********************************************************************* 
 
      SUBROUTINE LU2ENT(IP,KF1,KF2,PECM) 
 
C...Purpose: to store two partons/particles in their CM frame, 
C...with the first along the +z axis. 
      COMMON/LUJETS/N,K(4000,5),P(4000,5),V(4000,5) 
      COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200) 
      COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4) 
      SAVE /LUJETS/,/LUDAT1/,/LUDAT2/ 
 
C...Standard checks. 
      MSTU(28)=0 
      IF(MSTU(12).GE.1) CALL LULIST(0) 
      IPA=MAX(1,IABS(IP)) 
      IF(IPA.GT.MSTU(4)-1) CALL LUERRM(21, 
     &'(LU2ENT:) writing outside LUJETS memory') 
      KC1=LUCOMP(KF1) 
      KC2=LUCOMP(KF2) 
      IF(KC1.EQ.0.OR.KC2.EQ.0) CALL LUERRM(12, 
     &'(LU2ENT:) unknown flavour code') 
 
C...Find masses. Reset K, P and V vectors. 
      PM1=0. 
      IF(MSTU(10).EQ.1) PM1=P(IPA,5) 
      IF(MSTU(10).GE.2) PM1=ULMASS(KF1) 
      PM2=0. 
      IF(MSTU(10).EQ.1) PM2=P(IPA+1,5) 
      IF(MSTU(10).GE.2) PM2=ULMASS(KF2) 
      DO 110 I=IPA,IPA+1 
      DO 100 J=1,5 
      K(I,J)=0 
      P(I,J)=0. 
      V(I,J)=0. 
  100 CONTINUE 
  110 CONTINUE 
 
C...Check flavours. 
      KQ1=KCHG(KC1,2)*ISIGN(1,KF1) 
      KQ2=KCHG(KC2,2)*ISIGN(1,KF2) 
      IF(MSTU(19).EQ.1) THEN 
        MSTU(19)=0 
      ELSE 
        IF(KQ1+KQ2.NE.0.AND.KQ1+KQ2.NE.4) CALL LUERRM(2, 
     &  '(LU2ENT:) unphysical flavour combination') 
      ENDIF 
      K(IPA,2)=KF1 
      K(IPA+1,2)=KF2 
 
C...Store partons/particles in K vectors for normal case. 
      IF(IP.GE.0) THEN 
        K(IPA,1)=1 
        IF(KQ1.NE.0.AND.KQ2.NE.0) K(IPA,1)=2 
        K(IPA+1,1)=1 
 
C...Store partons in K vectors for parton shower evolution. 
      ELSE 
        K(IPA,1)=3 
        K(IPA+1,1)=3 
        K(IPA,4)=MSTU(5)*(IPA+1) 
        K(IPA,5)=K(IPA,4) 
        K(IPA+1,4)=MSTU(5)*IPA 
        K(IPA+1,5)=K(IPA+1,4) 
      ENDIF 
 
C...Check kinematics and store partons/particles in P vectors. 
      IF(PECM.LE.PM1+PM2) CALL LUERRM(13, 
     &'(LU2ENT:) energy smaller than sum of masses') 
      PA=SQRT(MAX(0.,(PECM**2-PM1**2-PM2**2)**2-(2.*PM1*PM2)**2))/ 
     &(2.*PECM) 
      P(IPA,3)=PA 
      P(IPA,4)=SQRT(PM1**2+PA**2) 
      P(IPA,5)=PM1 
      P(IPA+1,3)=-PA 
      P(IPA+1,4)=SQRT(PM2**2+PA**2) 
      P(IPA+1,5)=PM2 
 
C...Set N. Optionally fragment/decay. 
      N=IPA+1 
      IF(IP.EQ.0) CALL LUEXEC 
 
      RETURN 
      END