  We investigated the spiral spin density waves in the monolayer 1T-MnCl$_2$
for a set of spiral vectors based on first-principles calculations. The
magnetic ground states were evaluated by means of the generalized Bloch theorem
within the linear combination of pseudo-atomic orbitals. To reach our purpose,
a flat spiral configuration was constructed for the Mn magnetic atom by fixing
the direction of its magnetic moment. We confirmed that the ground state was a
spiral ground state. We also clarified that a phase transition from a spiral
ground state to the other ground states, such as the ferromagnetic state or the
antiferromagnetic state, appears when introducing the hole-electron doping.
Therefore, we justify that introducing the hole-electron doping tunes the phase
transition in the monolayer 1T-MnCl$_2$.
