Accuracy of effective core potential (ECP) is studied for two sizes of cores by density functional theory, Hartree-Fock and quantum Monte Carlo (QMC) methods using the MnO molecule as a test system. We compare the energy differences between high-spin and low-spin states that were previously found to be problematic for transition metal oxide solids calculations with ECPs. In order to disentangle errors caused by ECPs and by subsequent methods used in calculations, we construct a scalar-relativistic He-core ECP for Mn atom. We find that within high quality correlated calculations both Ne-core and He-core ECPs provide energy differences with comparable, high accuracy. (C) 2013 Elsevier B.V. All rights reserved.