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allen-tildesley-book
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README,
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older-version
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********************************************************************************
** FICHE F.4. VELOCITY VERSION OF VERLET ALGORITHM **
** This FORTRAN code is intended to illustrate points made in the text. **
** To our knowledge it works correctly. However it is the responsibility of **
** the user to test it, if it is to be used in a research application. **
********************************************************************************
C *******************************************************************
C ** TWO ROUTINES THAT TOGETHER IMPLEMENT VELOCITY VERLET METHOD. **
C ** **
C ** REFERENCE: **
C ** **
C ** SWOPE ET AL., J. CHEM. PHYS. 76, 637, 1982. **
C ** **
C ** ROUTINES SUPPLIED: **
C ** **
C ** SUBROUTINE MOVEA ( DT, M ) **
C ** MOVES POSITIONS AND PARTIALLY UPDATES VELOCITIES. **
C ** SUBROUTINE MOVEB ( DT, M, K ) **
C ** COMPLETES VELOCITY MOVE AND CALCULATES KINETIC ENERGY. **
C ** **
C ** PRINCIPAL VARIABLES: **
C ** **
C ** INTEGER N NUMBER OF MOLECULES **
C ** REAL DT TIMESTEP **
C ** REAL M ATOMIC MASS **
C ** REAL K KINETIC ENERGY **
C ** REAL RX(N),RY(N),RZ(N) POSITIONS **
C ** REAL VX(N),VY(N),VZ(N) VELOCITIES **
C ** REAL FX(N),FY(N),FZ(N) FORCES **
C ** **
C ** USAGE: **
C ** **
C ** AT THE START OF A TIMESTEP, MOVEA IS CALLED TO ADVANCE THE **
C ** POSITIONS AND 'HALF-ADVANCE' THE VELOCITIES. THEN THE FORCE **
C ** ROUTINE IS CALLED, AND THIS IS FOLLOWED BY MOVEB WHICH **
C ** COMPLETES THE ADVANCEMENT OF VELOCITIES. **
C *******************************************************************
SUBROUTINE MOVEA ( DT, M )
COMMON / BLOCK1 / RX, RY, RZ, VX, VY, VZ, FX, FY, FZ
C *******************************************************************
C ** FIRST PART OF VELOCITY VERLET ALGORITHM **
C ** **
C ** USAGE: **
C ** **
C ** THE FIRST PART OF THE ALGORITHM IS A TAYLOR SERIES WHICH **
C ** ADVANCES POSITIONS FROM T TO T + DT AND VELOCITIES FROM **
C ** T TO T + DT/2. AFTER THIS, THE FORCE ROUTINE IS CALLED. **
C *******************************************************************
INTEGER N
PARAMETER ( N = 108 )
REAL DT, M
REAL RX(N), RY(N), RZ(N)
REAL VX(N), VY(N), VZ(N)
REAL FX(N), FY(N), FZ(N)
INTEGER I
REAL DT2, DTSQ2
C *******************************************************************
DT2 = DT / 2.0
DTSQ2 = DT * DT2
DO 100 I = 1, N
RX(I) = RX(I) + DT * VX(I) + DTSQ2 * FX(I) / M
RY(I) = RY(I) + DT * VY(I) + DTSQ2 * FY(I) / M
RZ(I) = RZ(I) + DT * VZ(I) + DTSQ2 * FZ(I) / M
VX(I) = VX(I) + DT2 * FX(I) / M
VY(I) = VY(I) + DT2 * FY(I) / M
VZ(I) = VZ(I) + DT2 * FZ(I) / M
100 CONTINUE
RETURN
END
SUBROUTINE MOVEB ( DT, M, K )
COMMON / BLOCK1 / RX, RY, RZ, VX, VY, VZ, FX, FY, FZ
C *******************************************************************
C ** SECOND PART OF VELOCITY VERLET ALGORITHM **
C ** **
C ** USAGE: **
C ** **
C ** THE SECOND PART OF THE ALGORITHM ADVANCES VELOCITIES FROM **
C ** T + DT/2 TO T + DT. THIS ASSUMES THAT FORCES HAVE BEEN **
C ** COMPUTED IN THE FORCE ROUTINE AND STORED IN FX, FY, FZ. **
C *******************************************************************
INTEGER N
PARAMETER ( N = 108 )
REAL DT, M, K
REAL RX(N), RY(N), RZ(N)
REAL VX(N), VY(N), VZ(N)
REAL FX(N), FY(N), FZ(N)
INTEGER I
REAL DT2
C *******************************************************************
DT2 = DT / 2.0
K = 0.0
DO 200 I = 1, N
VX(I) = VX(I) + DT2 * FX(I) / M
VY(I) = VY(I) + DT2 * FY(I) / M
VZ(I) = VZ(I) + DT2 * FZ(I) / M
K = K + VX(I) ** 2 + VY(I) ** 2 + VZ(I) ** 2
200 CONTINUE
K = 0.5 * M * K
RETURN
END
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