Interface 3-XS

Summary:

The interface 3-XS is the standard interface for a 3-body system, where common blocks are used to pass data between the calling routine and the potential energy routine. The potential energy (and possibly the derivatives) are calculated for a single geometry for each call to POT. The subroutine POT requires no arguments. The common block PT1CM is required by the POT subroutine; this common block is used to pass the geometry of the triatomic system from the calling program to the potential routine and to return the electronic state energy and derivatives to the calling program. There are several other optional common blocks that may be used to further refine calls to POT. This interface may be used with or without derivatives.

Format and selected details:

SUBROUTINE POT

    COMMON /PT1CM/ R, ENERGY, DEDR
    DIMENSION R(3), DEDR(3)

Note: The following common blocks are optional and are used to pass
      control variables to the potential subroutine.
    COMMON /PT2CM/ NSURF, NDER, NFLAG
    COMMON /PT3CM/ LFLAG
    COMMON /PT4CM/ IPTPRT, IDUM
    COMMON /PT5CM/ EASYAB, EASYBC, EASYAC
    DIMENSION NFLAG(21), LFLAG(20), IDUM(19)

Note: The following common blocks are optional and are used to pass
      excited-state energies and derivatives if available.
    COMMON /PT6CM/ ENGY2, DE2DR
    COMMON /PT7CM/ ENGY12, DE12DR
    COMMON /PT8CM/ ENGY12
    COMMON /PT9CM/ D2E1D, D2E2D, D2E12D
    DIMENSION DE2DR(3), DE12DR(3), ENGY12(9), D2E1D(3,3), D2E2D(3,3), D2E12D(3,3)

    ENERGY  = SOME FUNCTION OF R . . . 
    DEDR(:) = . . . 

RETURN

END

Description of the common blocks:

The common block PT1CM is the only common block that is always required.
COMMON /PT1CM/ R, ENERGY, DEDR
DIMENSION R(3), DEDR(3)

R(3) input, double precision An array of the three internuclear bond distances. Atomic units are used.
ENERGY output, double precision The computed potential energy. Atomic units are used.

DEDR(3) output, double precision The computed derivatives with respect to the three internuclear distances. This field is included even when derivatives are not available. Atomic units are used.
The common block PT2CM is optional. It is used when there is more than one electronic state and/or derivatives are computed.
COMMON /PT2CM/ NSURF, NDER, NFLAG
DIMENSION NFLAG(21)

NSURF input, integer For a single surface routine, NSURF is not used. For a multiple surface routine, NSURF controls which surface is computed. When NSURF is equal to 0, the ground electronic state is used. When NSURF is equal to 1, the energy for the first excited electronic surface is computed. When both the ground and first excited surfaces are to be computed, NSURF is 2. Not all potential routines support NSURF=1 or 2.
NDER input, integer NDER controls the calculation of the derivatives of the energy with respect to the internuclear coordinates. If NDER is equal to 0, the derivatives are not calculated. If NDER is equal to 1, the energy and the first derivatives are calculated. If NDER is equal to 2, the energy and the first and second derivatives are calculated. Not all potential routines support NDER=1 or NDER=2. NDER controls the derivative calculations for whichever electronic surface or combination of surfaces is called for by NSURF. Therefore, if NSURF and NDER are both equal to 1, the energy and first derivatives for the first excited electronic state are computed. If NSURF is equal to 2 and NDER is equal to 1, the subprogram computes the energy and the derivatives for both the ground and the first excited electroinc states.
NFLAG(21) input, integer This is included for compatibility with existing codes. These flags may be used to control various options in the potential routine.
The common block PT3CM is optional.
COMMON /PT3CM/ LFLAG
DIMENSION LFLAG(20)

LFLAG(20) input, logical This is included for compatibility with existing codes. These flags may be used to control various options in the potential routine.
The common block PT4CM is optional and is used to control input/output.
COMMON /PT4CM/ IPTPRT, IDUM
DIMENSION IDUM(19)

IPTPRT input, integer This potential routine will write data to the file fort.IPTPRT.
IDUM(19) input, integer This potential routine will read data from zero or more of the files fort.IDUM(1), fort.IDUM(2), etc...
The common block PT5CM is optional.
COMMON /PT5CM/ EASYAB, EASYBC, EASYAC

EASYAB, EASYBC, EASYAC output, double precision The asymptotic energies at the minimum of the diatomic potential well.
The common block PT6CM is optional and is used to return excited surface energies and their derivatives when they are available.
COMMON /PT6CM/ ENGY2, DE2DR
DIMENSION DE2DR(3)

ENGY2, DE2DR(3) output, double precision The same as ENERGY and DEDR(3) (PT1CM) except for the first excited state. This common block is used with NSURF=1 or NSURF=2, where NSURF is set in PT2CM.
The common block PT7CM is optional and is used when a two-state system is represented in the diabatic representation.
COMMON /PT7CM/ ENGY12, DE12DR
DIMENSION DE12DR(3)

ENGY12, DE12DR(3) output, double precision The same as ENERGY and DEDR(3) (PT1CM) except for the diabatic coupling surface. This common block is used with NSURF=1 or NSURF=2, where NSURF is set in PT2CM.
The common block PT8CM is optional and is used when a two-state system is represented in the adiabatic representation.
COMMON /PT8CM/ ENGY12
DIMENSION ENGY12(8)

ENGY12(9) output, double precision The nine-dimension vector of the nonadiabatic coupling between the two electronic states.
The common block PT9CM is optional and is used when the potential routine is used to compute analytic Hessians.
COMMON /PT9CM/ D2E1D, D2E2D, D2E12D
DIMENSION D2E1D(3,3), D2E2D(3,3), D2E12D(3,3)

D2E1D(3,3) output, double precision The Hessian of the ground state.
D2E2D(3,3) output, double precision The Hessian of the first-excited state.
D2E12D(3,3) output, double precision The Hessian of the diabatic coupling surface.