--- /dev/null
+
+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