Adds angstrom to bohr conversion factor

Adds the angstrom to bohr conversion factor.

This conversion factor is required for the CREST adapter.

Author: calvinp0

arc/constants.pxd

@@ -1 +1 @@
-cdef double pi, Na, kB, R, h, hbar, c, e, m_e, m_p, m_n, amu, a0, E_h, F, E_h_kJmol, bohr_to_angstrom
+cdef double pi, Na, kB, R, h, hbar, c, e, m_e, m_p, m_n, amu, a0, E_h, F, E_h_kJmol, bohr_to_angstrom, angstrom_to_bohr

arc/constants.py

@@ -79,6 +79,7 @@
 epsilon_0 = 8.8541878128
 
 bohr_to_angstrom = 0.529177
+angstrom_to_bohr = 1.8897259886
 
 # Cython does not automatically place module-level variables into the module
 # symbol table when in compiled mode, so we must do this manually so that we
@@ -102,4 +103,5 @@
     'F': F,
     'epsilon_0': epsilon_0,
     'bohr_to_angstrom': bohr_to_angstrom,
+    'angstrom_to_bohr': angstrom_to_bohr,
 })