Excited_state_force_calculations using BerkeleyGW and Quantum ESPRESSO

Use the attached files as your sample input reference.

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Workflow for Running Calculations in the WFN_fi Directory

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1. Perform Fine Grid Calculation: pw.x < scf.in > scf.out

2. Conduct Phonon Calculation: ph.x < ph.in > ph.out

3. Convert Wavefunctions for BerkeleyGW: pw2bgw.x < pw2bgw.in > pw2bgw.out

4. (Optional) Perform Dynamic Matrix Calculation: dynmat.x < dynmat.in > dynmat.out

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Important Notes:

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• Number of Bands (nbnd): -> Ensure it includes the total number of valence and conduction bands used in the BSE calculation.

• Smearing: -> Apply smearing, even for semiconductors (suggested value: 1e-3 Ry), to allow ph.x to calculate electron-phonon (el-ph) coefficients.

• Critical Variables to Check: -> nogg=.true. -> electron_phonon='simple' -> fildvscf='NAME' -> trans=.true.

• Convergence: -> Typically, tr2_ph=1d-16 or 1d-18 is sufficient.

• Symmetry: -> Set search_sym=.false. to ensure that Quantum ESPRESSO computes the el-ph coefficients for all k-points.

• XML File Validation: -> Ensure that elph.xx.yy.xml files end with . If not, delete the last line to correct it. You can run the following script to fix these files: bash fix_elph_xml_files.bash

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Example Input for ph.x:

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phonon_calc $inputph verbosity = 'high' prefix = 'LiF' outdir = './' fildyn = 'dyn' fildvscf = 'dvscf' electron_phonon='simple' trans=.true. nogg=.true. tr2_ph=1.0d-18 search_sym=.false. / 0.0 0.0 0.0

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BerkeleyGW Workflow

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• Epsilon, Sigma, Kernel: -> Follow the usual steps.

• Absorption: -> Use the momentum operator. The polarization direction does not matter since the BSE solution is required. -> WFNq_fi is not needed when using the momentum operator.

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Excited State Forces Calculation

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To run the calculation: python3 /path_to_script/excited_forces.py

Input File (forces.inp) Variables:

• iexc = 1
• jexc = 1 # Generally, iexc = jexc. The code calculates <iexc | dH^bse | jexc>
• Calculate_Kernel = False # Set to False to avoid using the kernel.
• calc_IBL_way = False
• eqp_file = eqp.dat # QP energies on the fine grid (produced by the absorption calculation). Point to eqp.dat in the absorption directory.
• exciton_file = eigenvectors.h5
• acoutic_sum_rule = True
• el_ph_dir = ../../5-wfn_fi_6kpts/_ph0/MAPI.phsave/ # Directory where the el-ph coefficients are stored.
• just_real = True

Output:

• The calculated forces will be saved in forces_cart.out, containing the excited state forces for each atom in Cartesian coordinates, in units of eV/Å.

Important Note:

• The total force equals the sum of the excited state force and the DFT force.

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Expressing Forces in Phonon Displacement Basis

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1. Generate Displacement Patterns: dynmat.x < dynmat.in > dynmat.out

Example dynmat.in: $input fildyn='dyn' asr='crystal' fileig='eigvecs' filxsf='displacements.axsf' /

• The output file displacements.axsf will contain the displacement patterns for each phonon.

2. Convert Forces to Phonon Basis: python cart2ph_eigvec.py forces_cart.out displacements.axsf

• The output file forces_phonons_basis.out will contain the forces expressed in the phonon displacement basis. Here, forces_cart.out is the output of the excited state forces calculation.