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<p>We present a new routine for calculating the non-local thermodynamic equilibrium (non-LTE) 15 <span class="inline-formula">µ</span>m CO<span class="inline-formula">₂</span> cooling–heating of mesosphere and lower thermosphere in general circulation models. It uses the optimized models of the non-LTE in CO<span class="inline-formula">₂</span> for day and night conditions and delivers cooling–heating with an error not exceeding 1 K d<span class="inline-formula">⁻¹</span> even for strong temperature disturbances. The routine uses the accelerated lambda iteration and opacity distribution function techniques for the exact solution of the non-LTE problem and is about 1000 times faster than the standard matrix and line-by-line solution. It has an interface for feedbacks from the model and is ready for implementation. It may use any quenching rate coefficient of the CO<span class="inline-formula">₂</span>(<span class="inline-formula"><i>ν</i>₂</span>)<span class="inline-formula">+</span>O(<span class="inline-formula">³</span>P) reaction, handles large variations in O(<span class="inline-formula">³</span>P), and allows the user to vary the number of vibrational levels and bands to find a balance between the calculation speed and accuracy. The suggested routine can handle the broad variation in CO<span class="inline-formula">₂</span> both below and above the current volume mixing ratio, up to 4000 ppmv. This allows the use of this routine for modeling Earth's ancient atmospheres and the climate changes caused by increasing CO<span class="inline-formula">₂</span>.</p>