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eigvals

Computes the neigs algebraically smallest eigenvalues of a hermitian operator using the LOBPCG block eigensolver. In contrast to the Lanczos algorithm, LOBPCG reliably resolves excited states and their degeneracies, at the cost of a higher memory footprint. Only the eigenvalues are returned; use eigs to additionally obtain the eigenvectors.

The algorithm can be run either on-the-fly (matrix-free) or using a sparse matrix in the compressed-sparse-row format (see CSRMatrix).

Sources: sparse_diag.hpp ยท sparse_diag.cpp

Definition

On-the-fly

eigvals(ops::OpSum, block::Block, neigs::Int64; precision::Float64 = 1e-12,
        max_iterations::Int64 = 1000, random_seed::Int64 = 42)

arma::vec eigvals(OpSum const &ops, Block const &block, int64_t neigs,
                  double precision = 1e-12, int64_t max_iterations = 1000,
                  int64_t random_seed = 42);

Sparse matrix

eigvals(ops::CSRMatrix, block::Block, neigs::Int64; precision::Float64 = 1e-12,
        max_iterations::Int64 = 1000, random_seed::Int64 = 42)

template <typename idx_t, typename coeff_t>
arma::vec eigvals(CSRMatrix<idx_t, coeff_t> const &ops, Block const &block,
                  int64_t neigs, double precision = 1e-12,
                  int64_t max_iterations = 1000, int64_t random_seed = 42);

Parameters

Name Description Default
ops OpSum or CSRMatrix defining the bonds of the operator
block block on which the operator is defined
neigs number of (lowest) eigenvalues to compute
precision accuracy of the computed eigenvalues 1e-12
max_iterations maximum number of iterations 1000
random_seed random seed for setting up the initial block of vectors 42

Returns

An arma::vec (Julia: Vector{Float64}) holding the neigs lowest eigenvalues in ascending order.

Usage Example

block = Spinhalf(12)
ops = OpSum()
for i in 1:12
    ops += "J" * Op("SdotS", [i, mod1(i + 1, 12)])
end
ops["J"] = 1.0
eigenvalues = eigvals(ops, block, 3)

auto block = Spinhalf(12);
auto ops = OpSum();
for (int i = 0; i < 12; ++i) {
  ops += "J" * Op("SdotS", {i, (i + 1) % 12});
}
ops["J"] = 1.0;
arma::vec eigenvalues = eigvals(ops, block, 3);