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qmc/sse: default EPSILON to an ergodic value for sign-problematic models #98

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qmc/sse: default EPSILON to an ergodic value for sign-problematic models #98

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14946dd
qmc/sse: default EPSILON to an ergodic value for sign-problematic models
skilledwolf Jun 11, 2026
14408ae
Merge master into fix/sse-epsilon-ergodicity
egull Jul 21, 2026
5c03e07
Merge remote-tracking branch 'origin/master' into fix/sse-epsilon-erg…
skilledwolf Jul 22, 2026
f280192
qmc/sse: apply signed epsilon default before fallback
skilledwolf Jul 22, 2026
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39 changes: 21 additions & 18 deletions applications/qmc/sse4/model.h
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Original file line number Diff line number Diff line change
Expand Up @@ -90,19 +90,6 @@ class Model {
_lowering_matrix_elements.resize(lattice.max_site_type() + 1);

construct_vertices();

if (epsilon <= 0.0 && model.is_signed()) {
epsilon = *std::max_element(
_max_diag_me.begin(),
_max_diag_me.end()
);

std::cout
<< "Warning: Hamiltonian has a sign problem and EPSILON<=0.\n"
<< "Automatically setting EPSILON = "
<< epsilon
<< " (largest diagonal matrix element).\n";
}
}

std::vector<unsigned> const& nbstates() const
Expand Down Expand Up @@ -289,8 +276,7 @@ class Model {
unsigned nneighbors0 = lattice.nneighbors(sites[0]);
unsigned nneighbors1 = lattice.nneighbors(sites[1]);

// _max_diag_me[i] = std::numeric_limits<double>::min();
_max_diag_me[i] = std::numeric_limits<double>::lowest();
_max_diag_me[i] = std::numeric_limits<double>::lowest();

for (unsigned l = 0; l < nstates[i]; ++l) {
vertex_type vertex;
Expand Down Expand Up @@ -339,9 +325,26 @@ class Model {
if (epsilon == 0.0 && !have_diagonal)
throw std::runtime_error("Hamiltonian looks purely off-diagonal. "
"Parameter EPSILON has to be non zero for SSE to work.");

if (epsilon <= 0.0)
epsilon = 1e-6;
if (epsilon <= 0.0) {
if (model.is_signed()) {
// A tiny EPSILON can make the diagonal update effectively
// non-ergodic for signed models. Use the largest positive
// diagonal matrix element once those elements are known.
epsilon = 1e-6;
for (std::vector<double>::const_iterator it = _max_diag_me.begin();
it != _max_diag_me.end(); ++it)
if (*it > epsilon)
epsilon = *it;

std::cout
<< "Warning: Hamiltonian has a sign problem and EPSILON<=0.\n"
<< "Automatically setting EPSILON = " << epsilon
<< " (largest positive diagonal matrix element, with a "
"1e-6 minimum).\n";
} else {
epsilon = 1e-6;
}
}

std::set<unsigned>::const_iterator sti = site_types.begin();
for (; sti != site_types.end(); ++sti)
Expand Down
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