xamfX2C: Two-electon picture change corrections for the X2C Hamiltonian

This tutorial demonstrates the usage and usefulness of the (extended) atomic mean-field two-electron picture-change corrections for the X2C Hamiltonian. For more information on the actual details of the theory and implementation see Ref. [Knecht2022].

Methane (${\rm CH}_4$) - The ultrarelativistic case

In the following example, we will calculate total energies and spinor energies of CH $_4$ with several different Hamiltonians within the framework of DFT/PBE/dyall.v2z.

For convenience, in the Table below (see also Table IX in Ref. [Knecht2022]), we summarise the SCF total energy (E) and spinor energies ($\epsilon$) of the doubly degenerate occupied spinors for ${\rm CH}_4$ as obtained from DFT/PBE/v2z calculations within a four-component Dirac–Coulomb ($^4{\rm DC}$) as well as a two-component Hamiltonian framework, including the ${\rm (e)amfX2C}_{\rm DC}$ models. All energies are given in Hartree. The speed of light $c$ was reduced by a factor 10.

	${\rm 1eX2C}_{\rm D}$	${\rm AMFIX2C}_{\rm D}$	${\rm amfX2C}_{\rm DC}$	${\rm eamfX2C}_{\rm DC}$	${}^{4}{\rm DC}$
E	-42.14220	-42.14039	-42.26469	-42.25774	-42.25850
$\epsilon_1$	-10.22206	-10.22401	-10.36154	-10.36028	-10.35794
$\epsilon_2$	-0.65939	-0.65966	-0.66288	-0.66269	-0.66274
$\epsilon_3$	-0.35705	-0.35288	-0.35649	-0.35291	-0.35290
$\epsilon_{4-5}$	-0.33313	-0.33503	-0.33282	-0.33527	-0.33544

In order to make use of the (e)amf two-electron picture-change corrections, we will need to run atomic calculations for each element / nuclei type in the molecule. Hence, in the present case, we will run atomic calculations for C and H, respectively. The latter seems odd as H is by definition a one-electron system but as we will see, the MO coefficients will be needed for the ${\rm eamfX2C}_{\rm DC}$ model.

Discussion

For a detailed discussion of the performance of the various picture-change correction models for the X2C Hamiltonian in comparison with the four-component reference data, we refer the interested user to Ref. [Knecht2022]. Let us highlight, though, that for the particular case of ${\rm eamfX2C}_{\rm DC}$ all picture-change correction terms for DFT (and likewise HF) are derived in molecular basis on the basis of molecular densities which are built from a superposition of atomic input densities. Consequently, in contrast to the simpler ${\rm amfX2C}_{\rm DC}$ model, the former comprises atomic contributions regardless of the actual atom type, that is, both “light” (one-electron) and “heavy” (many-electron) atoms contribute equally. Bearing the latter in mind, the total SCF energies as well as spinor energies compiled in the above Table confirm the unique numerical performance of the ${\rm eamfX2C}_{\rm DC}$ Hamiltonian model in comparison to the ${}^4{\rm DC}$ reference data.

Atomic calculations

Note that, besides scaling the speed of light by a factor of $c$/10, we also use a point nucleus as well as a different set of constants as default in our calculations. This was done for Ref. [Knecht2022] to match calculations with ReSpect. Since we would like to reproduce in this tutorial the results from Ref. [Knecht2022], we will keep those details. Needless to say, the ${\rm eamf}$ model works for finite nuclei just as for point nuclei.

Warning

Do not use approximations like .LVCORR for the parent 4-component Hamiltonian. Always use .DOSSSS in the atomic molecular-mean field calculations.

H atom

For the atomic run of H, we use the following input (h.inp)

**DIRAC
.WAVE FUNCTION
**INTEGRAL
.NUCMOD
 1
*READIN
.UNCONTRACTED
**GENERAL
.CVALUE
13.703599907400d0
.CODATA
 CODATA10
**HAMILTONIAN
.X2Cmmf
.DOSSSS
.DFT
 PBE
.ONESYS
*X-AMFI
.amf
.eamf
**WAVE FUNCTION
.SCF
*SCF
**MOLECULE
*BASIS
.DEFAULT
 dyall.v2z
**END OF

and the xyz file (h.xyz):

1
atom xyz file
H          0.0000000000   0.0000000000   0.0000000000

and run the calculation with:

$ ./pam --inp=h.inp --mol=h.xyz --get="amfPCEC.h5" && mv amfPCEC.h5 amfPCEC.001.h5

In the latter step, we rename the file amfPCEC.h5 for later convenience.

C atom

For the atomic run of C, we use the following input (c.inp)

**DIRAC
.WAVE FUNCTION
**INTEGRAL
.NUCMOD
 1
*READIN
.UNCONTRACTED
**GENERAL
.CVALUE
13.703599907400d0
.CODATA
 CODATA10
**HAMILTONIAN
.X2Cmmf
.DOSSSS
.DFT
 PBE
*X-AMFI
.amf
.eamf
**WAVE FUNCTION
.SCF
*SCF
.CLOSED SHELL
4 0
.OPEN SHELL
1
2/0,6
**MOLECULE
*BASIS
.DEFAULT
 dyall.v2z
**END OF

and the xyz file (c.xyz):

1
atom xyz file
C          0.0000000000   0.0000000000   0.0000000000

and run the calculation with:

$ ./pam --inp=c.inp --mol=c.xyz --get="amfPCEC.h5" && mv amfPCEC.h5 amfPCEC.006.h5

In the latter step, we rename the file amfPCEC.h5 for later convenience.

Molecular calculations

Before embarking on the molecular calculations, we need to make a single preparatory step, that is:

$ $BUILD_DIR/merge_amf.py amfPCEC.006.h5 amfPCEC.001.h5

which will combine the atomic amfPCEC.xxx.h5 files into a single amfPCEC.h5 file ready for use in the molecular ${\rm (e)amfX2C}$ calculations below. The python script merge_amf.py is located in the build directory (named in the above code line $BUILD_DIR) of DIRAC.

The xyz coordinates for ${\rm CH}_4$ are given below (ch4.xyz):

5

C      0.0000000000   0.0000000000   0.0000000000
H      0.6298891440   0.6298891440  -0.6298891440
H      0.6298891440  -0.6298891440   0.6298891440
H     -0.6298891440   0.6298891440   0.6298891440
H     -0.6298891440  -0.6298891440  -0.6298891440

${\rm 1eX2C}_{\rm D}$

To run the molecular calculation for ${\rm CH}_4$ with the ${\rm 1eX2C}_{\rm D}$ Hamiltonian, that is, neglecting any two-electron picture-change corrections, we use the following input (1eX2C.inp):

**DIRAC
.WAVE FUNCTION
**GENERAL
.CVALUE
13.703599907400d0
.CODATA
 CODATA10
**INTEGRAL
.NUCMOD
 1
*READIN
.UNCONTRACTED
**HAMILTONIAN
.X2C
.DFT
 PBE
.NOAMFI
**WAVE FUNCTION
.SCF
*SCF
.CLOSED SHELL
 10
**MOLECULE
*BASIS
.DEFAULT
 dyall.v2z
**END OF