Abstract <p>In this contribution, the calculations of the total energy, correlation energy and equilibrium geometry of confined H₂ and HeH⁺ molecules are presented. The confinement effects are modeled using the repulsive external potential of cylindrical symmetry (harmonic oscillator potential). The computations are performed at the HF/d-aug-cc-pV6Z and CISD/d-aug-cc-pV6Z levels of theory. The results show that spatial confinement has a significant influence on the geometry and total energy of the investigated systems. The behavior of the correlation energy is more intricate. For both molecules, the absolute values of the correlation energy slightly increase as a function of confinement strength. On the other hand, the percentage share of correlation energy in the total energy (<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="214_2025_3209_Article_IEq1.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="48" /> </InlineMediaObject> <EquationSource Format="TEX">\(\%E_{\text {corr}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>%</mo> <msub> <mi>E</mi> <mtext>corr</mtext> </msub> </mrow> </math></EquationSource> </InlineEquation>) for H₂ reaches its maximum value near the critical frequency of the harmonic oscillator (<InlineEquation ID="IEq2"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="214_2025_3209_Article_IEq2.gif" Format="GIF" Height="14" Rendition="HTML" Resolution="72" Type="Linedraw" Width="56" /> </InlineMediaObject> <EquationSource Format="TEX">\(\omega \approx 1.3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ω</mi> <mo>≈</mo> <mn>1.3</mn> </mrow> </math></EquationSource> </InlineEquation> a.u.), at which the total energy becomes positive.</p> Graphical abstract <p></p>