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We introduce a new implementation of the coupled cluster method tailored bymatrix product states wave functions (DMRG-TCCSD), which employs the local pairnatural orbital approach (LPNO). By exploiting locality in the coupled clusterstage of the calculation, we were able to remove some of the limitations thathindered the application of the canonical version of the method to largersystems and/or with larger basis sets. We assessed the accuracy of theapproximation using two systems: tetramethyleneethane (TME) and oxo-Mn(Salen).Using the default cut-off parameters, we were able to recover over 99.7% and99.8% of canonical correlation energy for the triplet and singlet state of TMErespectively. In case of oxo-Mn(Salen), we found out that the amount ofretrieved canonical correlation energy depends on the size of the active space(CAS) - we retrieved over 99.6% for the larger 27 orbital CAS and over 99.8%for the smaller 22 orbital CAS. The use of LPNO-TCCSD allowed us to performthese calculations up to quadruple-$\zeta$ basis set amounting to 1178 basisfunctions. Moreover, we examined dependance of the ground state ofoxo-Mn(Salen) on CAS composition. We found out that the inclusion of 4d$_{xy}$orbital plays an important role in stabilizing the singlet state at theDMRG-CASSCF level via double-shell effect. However, by including dynamiccorrelation the ground state was found to be triplet regardless of the size ofthe basis set or composition of CAS, which is in agreement with previousfindings by canonical DMRG-TCCSD in smaller basis.

Towards the Efficient Local Tailored Coupled Cluster Approximation and the Peculiar Case of Oxo-Mn(Salen)