Construction of a unique metric in quasi-Hermitian quantum mechanics:   non-existence of the charge operator in a 2 x 2 matrix model
  For a specific exactly solvable 2 by 2 matrix model with a PT-symmetric Hamiltonian possessing a real spectrum, we construct all the eligible physical metrics and show that none of them admits a factorization CP in terms of an involutive charge operator C. Alternative ways of restricting the physical metric to a unique form are briefly discussed. 
Miloslav Znojil and Hendrik B. Geyer