The spectroscopic parameters of sodium cyanide, NaCN (X∼1A′), revisited

► NaCN is an abundant molecule in circumstellar shells of carbon-rich late-type stars. ► Recent newly measured lines turned out to be difficult to fit. ► Source of the problem was traced to incomplete diagonalization of the Hamiltonian. ► The experimental NaCN data have been fit better with fewer spectroscopic parameters. ► Earlier spectroscopic parameters for Na13CN have been revised.