# Example: FDE freeze-and-thaw: HeCO2¶

This example demonstrates how a freeze-and-thaw FDE calculation can be performed. As test system, a He-CO2 van der Waals complex is used. It will further be shown how different exchange-correlation potential can be used for different subsystems, and how different basis set expansions can be employed. For details, see C.R. Jacob, T.A. Wesolowski, L. Visscher, J. Chem. Phys. 123 (2005), 174104. It should be stressed that the basis set and integration grid used in this example are too small to obtain good results.

Summary:

• PW91 everywhere
• SAOP for He; PW91 for CO2
• FDE(s) calculation with PW91 everywhere

Important, this kind of FDE geometry optimization only works with the non-default STO pair fitting method. This means that one has to include the key STOFIT in the input for ADF. If one does not use the BASIS key, like in this example, one should also include the key STOFIT in all fragment calculations (also in the create runs).

PW91 everywhere

In the first part, the PW91 functional will be used for both the He and the CO2 subsystems. In this part, the FDE(m) basis set expansion is used, i.e., basis functions of the frozen subsystem are not included in the calculation of the nonfrozen subsystem.

First, the CO2 molecule is prepared. In this calculation, the C2v symmetry of the final complex is used, and the NOSYMFIT option has to be included because this molecule will be rotated as a frozen fragment.

$ADFBIN/adf << eor Title TEST 1 -- Preparation of frozen CO2 STOFIT Units Length Bohr end Atoms C 0.000000 0.000000 0.000000 O -2.192000 0.000000 0.000000 O 2.192000 0.000000 0.000000 end Symmetry C(2V) NOSYMFIT Fragments C t21.C O t21.O End integration 5.0 xc GGA pw91 end End Input eor mv TAPE21 t21.co2.0 Afterwards, the FDE calculation is performed. In this calculation, the He atom is the nonfrozen system, and the previously prepared CO2 molecule is used as frozen fragment. For this frozen fragment the RELAX option is specified, so that the density of this fragment is updated in freeze-and-thaw iteration (a maximum number of three iteration is specified).$ADFBIN/adf << eor
Title TEST 1 -- Embedding calculation: He + frozen CO2 density -- freeze-and-thaw
STOFIT

Units
Length Bohr
end

Atoms
He   0.000000  0.000000  6.019000 f=He
C    0.000000  0.000000  0.000000 f=co2
O   -2.192000  0.000000  0.000000 f=co2
O    2.192000  0.000000  0.000000 f=co2
end

Fragments
He   t21.He
co2  t21.co2.0  type=fde
fdeoptions RELAX
SubEnd
End

NOSYMFIT

integration 5.0

xc
GGA pw91
end

FDE
PW91K
FULLGRID
RELAXCYCLES 3
end

End Input
eor

SAOP for He; PW91 for CO2

In this second part, the above example is modified such that PW91 is employed for the CO2 subsystem, while the SAOP potential is used for He. This can be achieved by choosing SAOP in the XC key (this sets the functional that will be used for the nonfrozen subsystem). Additionally, for the frozen fragment the XC option is used to chose the PW91 functional for relaxing this fragment. Furthermore, the PW91 functional is chosen for the nonadditive exchange-correlation functional that is used in the embedding potential with the GGAPOTXFD and GGAPOTCFD options in the FDE key.

$ADFBIN/adf << eor Title TEST 2 -- Embedding calculation: He + frozen CO2 density -- freeze-and-thaw STOFIT Units Length Bohr end Atoms He 0.000000 0.000000 6.019000 f=He C 0.000000 0.000000 0.000000 f=co2 O -2.192000 0.000000 0.000000 f=co2 O 2.192000 0.000000 0.000000 f=co2 end Fragments He t21.He co2 t21.co2.0 type=fde fdeoptions RELAX XC GGA PW91 SubEnd End NOSYMFIT integration 5.0 xc MODEL SAOP end FDE PW91K FULLGRID GGAPOTXFD PW91x GGAPOTCFD PW91c RELAXCYCLES 3 end End Input eor FDE(s) calculation with PW91 everywhere In this third part, the PW91 functional is applied for both subsystems again, but in contrast to part 1, now the FDE(s) basis set expansion is used, i.e., the basis functions of the frozen subsystem are included in the calculation of the nonfrozen subsystem. This can be achieved by employing the USEBASIS option. This option can be combined with the RELAX option.$ADFBIN/adf << eor
Title TEST 3 -- Embedding calculation: He + frozen CO2 density -- freeze-and-thaw
STOFIT

Units
Length Bohr
end

Atoms
He   0.000000  0.000000  6.019000 f=He
C    0.000000  0.000000  0.000000 f=co2
O   -2.192000  0.000000  0.000000 f=co2
O    2.192000  0.000000  0.000000 f=co2
end

Fragments
He   t21.He
co2  t21.co2.0  type=fde
fdeoptions RELAX USEBASIS
SubEnd
End

NOSYMFIT

integration 5.0

xc
GGA pw91
end

FDE
PW91K
FULLGRID
RELAXCYCLES 3
end

End Input
eor
eor

The example continues with the same calculation where partly the SAOP potential is used.