[ALPS-users] Problem with DMRG and an Ising model

[Alexander Henry]

Dear ALPS,

I'm having some troubling getting a sensible result using DMRG with ALPS
2.3.0 on a simple transverse Ising model (i.e. using the "spin" model with
Jz != 0, Gamma != 0). Other methods seem to get the correct result with the
same parameters. It seems to be the Gamma term which causes the trouble.
I've pasted a script below which demonstrates the problem. The script gives
me the following output, with the second number being the ground state
energy found by each method:

Model fulldiag: {10.0: -5.3178022046739857}
Model sparsediag: {10.0: -5.3178022046739937}
Model mps_optim: {10.0: -5.3178022046739919}
Model dmrg: {10.0: -213.49032836522568}

Adjusting the various DMRG parameters produces similar results of E0 ~=
-210. I assume I am doing something wrong, or perhaps the Gamma term isn't
supported by DMRG? I have searched the mailing list archive without
success. Ultimately, I want to use DMRG on a more complex model which
includes a term similar to the Gamma / Sx term in the spin model.

Thank you for your great work,
Alex Henry


The script:

##########################
import pyalps
import numpy as np
import matplotlib.pyplot as plt
import copy
import math

models = ['fulldiag', 'sparsediag', 'mps_optim', 'dmrg']
parms = [{
        'LATTICE'    : "chain lattice",
        'MODEL'      : "spin",
        'local_S'    : 0.5,
        'Jxy'        : 0,
        'Jz'         : 1,
        'Gamma'      : 1,
        'h'          : 0,
        'NUMBER_EIGENVALUES' : 1,
        'L'          : 10,
    }]

def process(prefix, prog):
    input_file = pyalps.writeInputFiles(prefix, parms)
    res = pyalps.runApplication(prog, input_file)
    data =
pyalps.loadEigenstateMeasurements(pyalps.getResultFiles(prefix=prefix))
    return data

def get_results(d):
    E0 = {}
    for Lsets in d:
        L = pyalps.flatten(Lsets)[0].props['L']
        allE = []
        for q in pyalps.flatten(Lsets):
            allE += list(q.y)
        allE = np.sort(allE)
        E0[L] = allE[0]
    return E0

results = {}
for model in models:
    name = "ising_" + model
    data = process(name, model)
    results[model] = (get_results(data))

for model in models:
    print "Model " + model + ": " + str(results[model])
#############################
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