# Example: Bond Energy analysis open-shell fragments: PCCP¶

Download PCCP_Unr_BondEnergy.run

#! /bin/sh

# This example illustrates advanced usage of the bond energy decomposition

# A proper decomposition of an electron-pair bond energy requires specifying
# opposite spins for the unpaired electrons of the respective radical fragments,
# which can be done with the input key FragOccupations. The specified alpha- and
# beta-spin configurations of the radical fragments are shown in the output
# section B U I L D.

# Please note that if one neglects explicitly specifying opposite spins for the
# unpaired electrons of the fragments, each of them is treated as being half an
# alpha and half a beta electron and consequently, they enter into a spurious
# Pauli repulsive interaction. This results, among others, into the Pauli
# repulsion term being too repulsive and the orbital interaction term being too
# much stabilizing.

# The example consists of an analysis of the C-C single bond between two CP
# radicals in the four-atomic molecule PCCP. The CP fragment calculations used
# to provide the TAPE21 for the overall PCCP calculation must be done, for
# technical reasons, in the restricted mode ('cp_fpccp_asr'). The proper spins
# are then specified in the calculation of the overall molecule using the
# FragOccupations key ('pccp_fa1_as'). Note that this implies a slight
# approximation because the bond energy computed in this way refers to the
# energy difference between closed-shell PCCP and two CP radicals that are
# described by orbitals from a spin-restricted SCF calculation, which have been
# given an unrestricted occupation. In other words, the set of alpha- and beta-
# spin orbitals are identical and the effect of spin polarization is missing. In
# practice, this leads to minor energy differences with respect to the correct
# bond energy, that is, the energy difference between closed-shell PCCP and two
# CP radicals treated in the unrestricted mode, i.e., for which the set of
# alpha- and beta-spin orbitals are allowed to relax toward different solutions
# in the SCF procedure. This correction term can be computed directly by
# carrying out an unrestricted computation of the CP radical ('cp_fpccp_asu') using the
# restricted CP radical ('cp_fpccp_asr') as a fragment.

# The pure orbital interaction effect of forming the electron bonding combination
# of the two radicals can be isolated from the full orbital interaction by carrying out a
# separate calculation. In this calculation (pccp_fa1_pb) the bond energy analysis is performed in
# the absence of any virtual CP fragment orbitals, using the key REMOVEFRAGORBITALS.

$ADFBIN/adf<<eor TITLE cp_fpccp_asr EPRINT SFO eig ovl END XC GGA BECKE PERDEW END ATOMS C .0000 .0000 .6681 P .0000 .0000 2.2555 END BASIS Type TZ2P Core Large END NumericalQuality Good eor rm logfile mv TAPE21 t21cp_fpccp$ADFBIN/adf<<eor
TITLE cp_fpccp_asu
EPRINT
SFO eig ovl
END
XC
GGA  BECKE PERDEW
END
ATOMS
C         .0000    .0000    .6681  f=CP
P         .0000    .0000   2.2555  f=CP
END
FRAGMENTS
CP   t21cp_fpccp
END
FRAGOCCUPATIONS
CP
SIGMA 3//2
PI    2//2
SUBEND
END
UNRESTRICTED
CHARGE     0   1
NumericalQuality Good
eor

rm TAPE21 logfile

$ADFBIN/adf<<eor TITLE pccp_fa1_pb EPRINT SFO eig ovl ORBPOP 20 20 SUBEND END XC GGA BECKE PERDEW END ATOMS P .0000 .0000 2.2555 f=CP_A C .0000 .0000 .6681 f=CP_A C .0000 .0000 -.6681 f=CP_B P .0000 .0000 -2.2555 f=CP_B END NumericalQuality Good FRAGMENTS CP_A t21cp_fpccp CP_B t21cp_fpccp END SYMMETRY C(LIN) RemoveAllFragVirtuals FRAGOCCUPATIONS CP_A SIGMA 3//2 PI 2//2 SUBEND CP_B SIGMA 2//3 PI 2//2 SUBEND END eor rm logfile mv TAPE21 PCCPpb.t21$ADFBIN/adf<<eor
TITLE pccp_fa1_as
EPRINT
SFO eig ovl
ORBPOP  20  20
SUBEND
END
XC
GGA  BECKE PERDEW
END
ATOMS
P         .0000    .0000   2.2555  f=CP_A
C         .0000    .0000    .6681  f=CP_A
C         .0000    .0000   -.6681  f=CP_B
P         .0000    .0000  -2.2555  f=CP_B
END
NumericalQuality Good
FRAGMENTS
CP_A   t21cp_fpccp
CP_B   t21cp_fpccp
END
SYMMETRY   C(LIN)
FRAGOCCUPATIONS
CP_A
SIGMA 3//2
PI    2//2
SUBEND
CP_B
SIGMA 2//3
PI    2//2
SUBEND
END
eor

rm logfile
mv TAPE21 PCCP.t21

$ADFBIN/adf<<eor TITLE B3LYP restricted XC Hybrid B3LYP End Atoms C .0000 .0000 .6681 P .0000 .0000 2.2555 End Basis Type TZ2P Core None END NumericalQuality Good eor rm logfile mv TAPE21 CP.t21$ADFBIN/adf<<eor
TITLE B3LYP unrestricted
XC
Hybrid B3LYP
End

Atoms
C         .0000    .0000    .6681  f=CP
P         .0000    .0000   2.2555  f=CP
End
Fragments
CP   CP.t21
End
Unrestricted
Charge 0 1
Fragoccupations
CP
SIGMA 7//6
PI    4//4
Subend
End
NumericalQuality Good
eor

rm TAPE21 logfile

$ADFBIN/adf<<eor TITLE PCCP B3LYP PAIRBONDING XC Hybrid B3LYP End Atoms P .0000 .0000 2.2555 f=CP_A C .0000 .0000 .6681 f=CP_A C .0000 .0000 -.6681 f=CP_B P .0000 .0000 -2.2555 f=CP_B End NumericalQuality Good Fragments CP_A CP.t21 CP_B CP.t21 End RemoveAllFragVirtuals Fragoccupations CP_A SIGMA 7//6 PI 4//4 Subend CP_B SIGMA 6//7 PI 4//4 Subend End eor rm logfile mv TAPE21 PCCPpb_hybrid.t21$ADFBIN/adf<<eor
TITLE PCCP B3LYP
XC
Hybrid B3LYP
End

Atoms
P         .0000    .0000   2.2555  f=CP_A
C         .0000    .0000    .6681  f=CP_A
C         .0000    .0000   -.6681  f=CP_B
P         .0000    .0000  -2.2555  f=CP_B
End
NumericalQuality Good
Fragments
CP_A   CP.t21
CP_B   CP.t21
End
Fragoccupations
CP_A
SIGMA 7//6
PI    4//4
Subend
CP_B
SIGMA 6//7
PI    4//4
Subend
End
eor

rm logfile
mv TAPE21 PCCPhybrid.t21