better fix for #4

Author: pierre-24

phonopy_vibspec/phonons_analyzer.py

@@ -13,6 +13,8 @@
     from phonopy.units import VaspToCm  # noqa
 except ImportError:
     from phonopy.physical_units import get_physical_units
+    physical_units = get_physical_units()
+    VaspToCm = physical_units.DefaultToTHz * physical_units.THzToCm
 
 from phonopy_vibspec.spectra import RamanSpectrum, InfraredSpectrum
 from phonopy_vibspec.vesta import VestaVector, make_vesta_file
@@ -24,8 +26,6 @@
 
 HUGE_MASS = 10000  # AMU
 
-physical_units = get_physical_units()
-
 l_logger = logger.getChild(__name__)
 
 
@@ -70,7 +70,7 @@ def __init__(
         self.N = self.structure.get_number_of_atoms()
         l_logger.info('Analyze {} modes (including acoustic)'.format(3 * self.N))
         self.frequencies = numpy.sqrt(numpy.abs(eigv.real)) * numpy.sign(eigv.real)
-        self.frequencies *= physical_units.DefaultToTHz * physical_units.THzToCm  # in [cm⁻¹]
+        self.frequencies *= VaspToCm  # in [cm⁻¹]
 
         if self.frequencies[0] < -30:
             l_logger.warn('The first frequency is very small: {:.3f} cm⁻¹'.format(self.frequencies[0]))