# Cheat sheet#

Here we provide quick reference calculation for some of the most common X-ray spectrum calculations, considering XPS/IE, XAS, and (non-resonant) XES of water.

## IEs and XPS#

### Koopmans’ theorem#

While it is not recommended for any production calculations, estimates of ionization energies can be obtained from Koopmans’ theorem:

import numpy as np
import veloxchem as vlx

# for vlx
silent_ostream = vlx.OutputStream(None)
from mpi4py import MPI

comm = MPI.COMM_WORLD
# au to eV conversion factor
au2ev = 27.211386

water_xyz = """
O       0.0000000000     0.0000000000     0.1178336003
H      -0.7595754146    -0.0000000000    -0.4713344012
H       0.7595754146     0.0000000000    -0.4713344012
"""

# Create veloxchem mol and basis objects
bas_vlx = vlx.MolecularBasis.read(mol_vlx, "6-31G")

# Perform SCF calculation
scf_gs = vlx.ScfRestrictedDriver(comm, ostream=silent_ostream)
scf_gs.compute(mol_vlx, bas_vlx)

# Extract orbital energies
orbital_energies = scf_gs.scf_tensors["E"]
print("1s E from Koopmans' theorem:", np.around(au2ev * orbital_energies, 2), "eV")

* Warning * Environment variable OMP_NUM_THREADS not set.
* Warning * Setting OMP_NUM_THREADS to 4.
* Warning * Setting MKL_THREADING_LAYER to "GNU".
1s E from Koopmans' theorem: -559.5 eV


### $$\Delta$$-methods#

Substantially improved ionization energies are obtained using $$\Delta$$-methods, where the energy difference of the ground state and core-hole state is used to estimate the IE:

import copy

import numpy as np
from pyscf import gto, mp, scf

water_xyz = """
O       0.0000000000     0.0000000000     0.1178336003
H      -0.7595754146    -0.0000000000    -0.4713344012
H       0.7595754146     0.0000000000    -0.4713344012
"""

# Create pyscf mol object
mol = gto.Mole()
mol.atom = water_xyz
mol.basis = "6-31G"
mol.build()

# Perform unrestricted SCF calculation
scf_gs = scf.UHF(mol)
scf_gs.kernel()

# Copy molecular orbitals and occupations
mo0 = copy.deepcopy(scf_gs.mo_coeff)
occ0 = copy.deepcopy(scf_gs.mo_occ)

# Create 1s core-hole by setting alpha_0 population to zero
occ0 = 0.0

# Perform unrestricted SCF calculation with MOM constraint
scf_ion = scf.UHF(mol)
scf_ion.kernel()

# Run MP2 on neutral and core-hole state
mp_res = mp.MP2(scf_gs).run()
mp_ion = mp.MP2(scf_ion).run()

# IE from energy difference
print(
"HF ionization energy:",
np.around(au2ev * (scf_ion.energy_tot() - scf_gs.energy_tot()), 2),
"eV",
)
print(
"MP2 ionzation energy:", np.around(au2ev * (mp_ion.e_tot - mp_res.e_tot), 2), "eV"
)

converged SCF energy = -75.9838703827192  <S^2> = 6.3353767e-12  2S+1 = 1

Overwritten attributes  get_occ  of <class 'pyscf.scf.uhf.UHF'>

converged SCF energy = -56.0754789470865  <S^2> = 0.76257805  2S+1 = 2.0125387
E(UMP2) = -76.1130483955489  E_corr = -0.129178012829713
E(UMP2) = -56.1523709631035  E_corr = -0.076892016016965
HF ionization energy: 541.73 eV
MP2 ionzation energy: 543.16 eV


## XAS#

Absorption spectra can be calculated using CVS-ADC:

import gator
import matplotlib.pyplot as plt

water_xyz = """
O       0.0000000000     0.0000000000     0.1178336003
H      -0.7595754146    -0.0000000000    -0.4713344012
H       0.7595754146     0.0000000000    -0.4713344012
"""

# Construct structure and basis objects
struct = gator.get_molecule(water_xyz)
basis = gator.get_molecular_basis(struct, "6-31G")

# Perform SCF calculation
scf_gs = gator.run_scf(struct, basis)

# Calculate the 6 lowest eigenstates with CVS restriction to MO #1 (oxygen 1s)
struct, basis, scf_gs, method="cvs-adc2x", singlets=4, core_orbitals=1
)

# Print information on eigenstates

plt.figure(figsize=(6, 5))
# Convolute using functionalities available in gator and adcc
plt.show()

+--------------------------------------------------------------+
| cvs-adc2x                               singlet ,  converged |
+--------------------------------------------------------------+
|  #        excitation energy     osc str    |v1|^2    |v2|^2  |
|          (au)           (eV)                                 |
|  0      19.71638      536.5099   0.0175       0.8       0.2  |
|  1       19.7967      538.6956   0.0368    0.8087    0.1913  |
|  2      20.49351      557.6567   0.0098    0.7858    0.2142  |
|  3      20.50482      557.9647   0.1007    0.8441    0.1559  |
+--------------------------------------------------------------+ • To be added

## XES#

The non-resonant X-ray emission spectrum can be calculated with a two-step approach using ADC:

import copy

import matplotlib.pyplot as plt
import numpy as np
from pyscf import gto, mp, scf

water_xyz = """
O       0.0000000000     0.0000000000     0.1178336003
H      -0.7595754146    -0.0000000000    -0.4713344012
H       0.7595754146     0.0000000000    -0.4713344012
"""

# Create pyscf mol object
mol = gto.Mole()
mol.atom = water_xyz
mol.basis = "6-31G"
mol.build()

# Perform unrestricted SCF calculation
scf_res = scf.UHF(mol)
scf_res.kernel()

# Copy molecular orbitals
mo0 = copy.deepcopy(scf_res.mo_coeff)
occ0 = copy.deepcopy(scf_res.mo_occ)

# Create 1s core-hole by setting alpha_0 population to zero
occ0 = 0.0

# Perform unrestricted SCF calculation with MOM constraint
scf_ion = scf.UHF(mol)
scf_ion.kernel()

# Perform ADC calculation

converged SCF energy = -75.9838703827193  <S^2> = 6.3380412e-12  2S+1 = 1
converged SCF energy = -56.0754789470865  <S^2> = 0.76257805  2S+1 = 2.0125387
Starting adc2  Jacobi-Davidson ...
Niter n_ss  max_residual  time  Ritz values
1     8       0.77768   1.8s  [-19.29082067 -19.19940205 -18.96181907 -18.3637012 ]
2    16     0.0010764  219ms  [-19.44427093 -19.36689642 -19.16826546 -18.57947501]
3    24    4.4301e-07  582ms  [-19.44429764 -19.3669284  -19.16831914 -18.57954987]
=== Converged ===
Number of matrix applies:    24
Total solver time:             2s 656ms

# Print information on eigenstates

plt.figure(figsize=(6, 5))
# Convolute using functionalities available in gator and adcc
plt.show()

+--------------------------------------------------------------+
| adc2                                        any ,  converged |
+--------------------------------------------------------------+
|  #        excitation energy     osc str    |v1|^2    |v2|^2  |
|          (au)           (eV)                                 |
|  0      -19.4443     -529.1063   0.0568    0.9548    0.0452  |
|  1     -19.36693      -527.001   0.0458    0.9503   0.04966  |
|  2     -19.16832     -521.5965   0.0419    0.9376   0.06245  |
|  3     -18.57955     -505.5753   0.0031    0.9335   0.06655  |
+--------------------------------------------------------------+ ### Ground state MOs#

To be added, using the approach here.