*Methods.* Calculations were carried out using a mixed quantum-classical theory approach that is implemented in the code MQCT. The large basis set of rotational states used in these calculations permits us to predict thermally averaged cross sections for 441 transitions in para- and ortho-H_{2}O in a broad range of temperatures.

*Results.* It is found that all state-to-state transitions in the H_{2}O + H_{2}O system split into two well-defined groups, one with higher cross-section values and lower energy transfer, which corresponds to the dipole-dipole driven processes. The other group has smaller cross sections and higher energy transfer, driven by higher-order interaction terms. We present a detailed analysis of the theoretical error bars, and we symmetrized the state-to-state transition matrixes to ensure that excitation and quenching processes for each transition satisfy the principle of microscopic reversibility. We also compare our results with other data available from the literature for H_{2}O + H_{2}O collisions.

*Methods*. We developed and applied a new general method that allows the generation of rate coefficients for excitation and quenching processes that automatically satisfy the principle of microscopic reversibility and also helps to cover the range of low collision energies by interpolation of cross sections between the process threshold and the computed data points.

*Results*. We find that in the range of intermediate temperatures, 150 < *T* < 600 K, our new rate coefficients are in good agreement with those reported earlier, but for higher temperatures, 600 < *T* < 1000 K, the new revised temperature dependence is recommended. The low temperature range, 5 < *T* < 150 K, is now covered by the above-mentioned interpolation of cross sections down to the process threshold.