Dear All,

My question is related to the calculated results of the below code. This entails MPS implantation to reach energy gap of a Heisenberg chain in the presence of a magnetic field.

We wrote:

%%%%%%%%%%%%%%% Start %%%%%%%%%%%%%%%%%%%%%%%

import pyalps
import numpy as np
import matplotlib.pyplot as plt
import pyalps.plot

parms = []
for hz in [0.0, 0.2, 0.4, 0.6, 0.8, 1.0]:
     parms.append(
         {
           'LATTICE'        : "open chain lattice",
           'MODEL'          : "spin",
           'CONSERVED_QUANTUMNUMBERS' : 'N,Sz',
           'Sz_total'                  : 0,
           'J'                             : 1,
           'h'                            : hz,
           'SWEEPS'               : 4,
           'L'                            : 32,
           'MAXSTATES'        : 100,
          'NUMBER_EIGENVALUES'         : 2
         }
     )

input_file = pyalps.writeInputFiles('parm_spin_one_half_gap',parms)
res = pyalps.runApplication('mps_optim',input_file,writexml=True)

data = pyalps.loadEigenstateMeasurements(pyalps.getResultFiles(prefix='parm_spin_one_half_gap'))

energies = np.empty(0)
for s in data[0]:
    if s.props['observable'] == 'Energy':
        energies = s.y
    else:
        print(s.props['observable'], ':', s.y[0])
energies.sort()
print('Energies:', end=' ')
for e in energies:
    print(e, end=' ')
print('\nGap:', abs(energies[1]-energies[0]))

%%%%%%%%%%%%%%% END %%%%%%%%%%%%%%%%%%%%%%%

Although, in the file "parm_spin_one_half_gap.in.xml" there are expensive information such as energies for any "hz", we need to collect all results in a *.dat or *.txt file.

For instance, a "file.txt" as:

hz gap

0.0 0.11

0.2 0.12

0.4 0.14

0.6 0.15

How can we generate such a file?!