Which electronic state is the ground state of methylene?

The ground state of methylene ($C{H}_2$) is the triplet state.1

In this state, the two nonbonding electrons occupy different orbitals and have parallel spins, which results in a magnetic moment. Experimental evidence from magnetic resonance spectroscopy confirms this configuration, showing a bond angle of 136°. While methylene can exist in a singlet state—where electrons are paired in the same orbital—this form is higher in energy and has a smaller bond angle of 104°. Generally, carbenes with only carbon or hydrogen substituents, such as methylene, are characterized by a triplet ground state.1