We propose a facile approach for optimizing the magnetic field sweep, based on the knowledge of the fixed-field dependence of the polarization transfer efficiency. As a model system 13 C1-allyl pyruvate is chosen, since the PHIP transfer was discussed in detail above. First, the field dependence of C polarization is calculated in experiments with hydrogenation at fixed fields. To describe this calculation, we first define the Hamiltonian at arbitrary low field B as follows: