We compare the three-dimensional gas temperature distributions obtained by a dedicated radiative transfer and photoionization code, mocassin, against those obtained by the recently developed smooth particle hydrodynamics (SPH) plus ionization code iVINE for snapshots of a hydrodynamical simulation of a turbulent interstellar medium (ISM) irradiated by a nearby O star. Our tests demonstrate that the global ionization properties of the region are correctly reproduced by iVINE, hence validating further application of this code to the study of feedback in star-forming regions. However, we highlight potentially important discrepancies in the detailed temperature distribution. In particular, we show that in the case of highly inhomogeneous density distributions, the commonly employed on-the-spot (OTS) approximation yields unrealistically sharp shadow regions which can affect the dynamical evolution of the system. We implement a simple strategy to include the effects of the diffuse field in future calculations, which makes use of physically motivated temperature calibrations of the diffuse-field-dominated regions and can be readily applied to similar codes. We find that while the global qualitative behaviour of the system is captured by simulations with the OTS approximation, the inclusion of the diffuse field in iVINE (called DiVINE) results in a stronger confinement of the cold gas, leading to denser and less coherent structures. This in turn leads to earlier triggering of star formation. We confirm that turbulence is being driven in simulations that include the diffuse field, but the efficiency is slightly lower than in simulations that use the OTS approximation.