  Understanding the physical processes that determine the relaxation $T_{1}$
and dephasing $T_2$ times of molecular spin qubits is critical for envisioned
applications in quantum metrology and information processing. Recent spin-echo
$T_1$ measurements of solid-state molecular spin qubits have stimulated the
development of quantum mechanical models for predicting intrinsic spin qubit
timescales using first-principles electronic structure methods. We develop an
alternative semi-empirical approach to construct Redfield quantum master
equations for molecular spin qubits using a stochastic Haken-Strobl model for a
central spin with a fluctuating gyromagnetic tensor due to spin-lattice
interaction and a fluctuating local magnetic field due to interactions with
other lattice spins. Using a vanadium-based spin qubit as a case study, we
compute qubit population and decoherence timescales as a function of
temperature and magnetic field using a bath spectral density parametrized with
a small number of $T_{1}$ measurements. The theory quantitatively agrees with
experimental data over a range of conditions beyond those used to parametrize
the model, demonstrating the generalization potential of the method. The
ability of the model to describe the temperature dependence of the ratio
$T_2/T_1$ is discussed and possible applications for designing novel
molecule-based quantum magnetometers are suggested.
