!!!!!!!!!!!!!!!!!!!!!!!!!!! Program 7.2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! !
! Please Note: !
! !
! (1) This computer program is written by Tao Pang in conjunction with !
! his book, "An Introduction to Computational Physics," published !
! by Cambridge University Press in 1997. !
! !
! (2) No warranties, express or implied, are made for this program. !
! !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
MODULE CSEED
INTEGER :: ISEED
END MODULE CSEED
!
SUBROUTINE MXWL (N,M,T,V)
!
! This subroutine assigns velocities according to the Maxwell distribution.
! N is the total number of velocity components and M is the total number of
! degrees of freedom. T is the system temperature in the reduced units.
! Copyright (c) Tao Pang 1997.
!
IMPLICIT NONE
INTEGER, INTENT (IN) :: N,M
INTEGER :: I
REAL, INTENT (IN) :: T
REAL :: V1,V2,EK,VS
REAL, INTENT (OUT), DIMENSION (N) :: V
!
! Assign a Gaussian distribution to each velocity component
!
DO I = 1, N-1, 2
CALL GRNF (V1,V2)
V(I) = V1
V(I+1) = V2
END DO
!
! Scale the velocity to satisfy the partition theorem
!
EK = 0.0
DO I = 1, N
EK = EK+V(I)*V(I)
END DO
VS = SQRT(EK/(M*T))
DO I = 1, N
V(I) = V(I)/VS
END DO
END SUBROUTINE MXWL
!
SUBROUTINE GRNF (X,Y)
!
! Two Gaussian random numbers generated from two uniform random
! numbers. Copyright (c) Tao Pang 1997.
!
IMPLICIT NONE
REAL, INTENT (OUT) :: X,Y
REAL :: PI,R1,R2,R,RANF
!
PI = 4.0*ATAN(1.0)
R1 = -ALOG(1.0-RANF())
R2 = 2.0*PI*RANF()
R1 = SQRT(2.0*R1)
X = R1*COS(R2)
Y = R1*SIN(R2)
END SUBROUTINE GRNF
!
FUNCTION RANF() RESULT (R)
!
! Uniform random number generator x(n+1) = a*x(n) mod c with
! a=7**5 and c = 2**(31)-1. Copyright (c) Tao Pang 1997.
!
USE CSEED
IMPLICIT NONE
INTEGER :: IH,IL,IT,IA,IC,IQ,IR
DATA IA/16807/,IC/2147483647/,IQ/127773/,IR/2836/
REAL :: R
!
IH = ISEED/IQ
IL = MOD(ISEED,IQ)
IT = IA*IL-IR*IH
IF(IT.GT.0) THEN
ISEED = IT
ELSE
ISEED = IC+IT
END IF
R = ISEED/FLOAT(IC)
END FUNCTION RANF