Fast Randomized Iteration for Electronic Structure
An implementation of FRI methods for quantum chemistry

The FRIES_bin directory contains source code for several different methods within the FCIFRI/FCIQMC framework as applied to different types of systems.
Executables that perform calculations on molecular systems (fciqmc_mol.c, frimulti_mol.c, frisys_mol.c) use the output from a HartreeFock calculation to calculate Hamiltonian matrix elements. The hf_path
input argument specifies the path to the directory that contains these files. The HF orbitals must be ordered the same in the eris.txt, hcore.txt, and symm.txt files. These files must be named and formatted as follows:
n_elec
: The total number of electrons in the systemn_frozen
: Number of core electrons to freeze in the calculation. The orbitals with the lowest indices, as determined by the ordering in the above files, will be frozen.eps
: The value of \( \varepsilon \) (the imaginary time step) to use to evolve the solution vector during the calculationhf_energy
: The HF electronic energy of the system, in Hartrees (optional, but useful for accelerating convergence)Code for running calculations on the Hubbard model (fciqmc_hh.c and frisys_hh.c) use the parameters in the file at the path specified by the params_path
input argument. See the examples for an example format of this file. The parameters are as follows:
n_elec
: The total number of electrons in the systemlat_len
: The number of lattice sites along one dimensionn_dim
: The number of dimensions in the latticeeps
: The value of \( \varepsilon \) (the imaginary time step) to use to evolve the solution vector during the calculationU
: The onsite repulsion energy, in units of \( t \) (the kinetic energy hoping term)hf_energy
: The HF electronic energy of the system, in Hartrees (optional, but useful for accelerating convergence)All executables produce the following files in the directory specified by the result_dir
input parameter:
projnum.txt
: A list of the values \( \langle \phi_\text{T}  \hat{H}  \psi^{(i)} \rangle \) calculated at each iteration in the trajectory, where \( \phi_\text{T} \) is the trial wave functionprojden.txt
: A list of the values \( \langle \phi_\text{T}  \psi^{(i)} \rangle \) calculated at each iterationS.txt
: The value of the energy shift after every 10 iterations in the trajectoryparams.txt
: A summary of the input parameters for this calculationExecutables for FCIQMC calculations produce these additional output files:
N.txt
: The number of walkers after each iterationnnonz.txt
: The number of nonzero vector elements after each iterationExecutables for FCIFRI calculations produce one additional output file:
norm.txt
: The vector onenorm calculated after each iteration