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M. Lundberg and M. G. Delcey
Yet, the X-ray modeling field is still evolving rapidly. This is certainly true for the
multiconfigurational approach, with new method developments constantly shaping
the way these calculations are performed. This process is likely to continue and
already now, many developments in related fields offer great promises to lift some of
the main limitations of the method. As an example, the CPP approach has been applied
to many wavefunction models to efficiently compute the spectrum at any energy
range [22]. While not available yet, an efficient CPP-CAS/RAS implementation
would alleviate the cost associated with the high density of states. It could also
provide higher accuracy by, for example, allowing core relaxation and ensuring
better consistency between different calculations by removing artifacts caused by
state averaging.
Similarly, while multiconfigurational simulations have mainly been limited to a
single metal atom because of active-space restrictions, many techniques have been
developed recently to push this limit, e.g., the density matrix renormalization group
[15], full CI quantum Monte Carlo [12], or Heat-Bath CI [37]. While those methods
still have not been used to compute X-ray spectra, and some technical difficulties are
still left to be overcome, the potential to calculate X-ray spectra of some of the fascinating natural and synthetic multi-metallic complexes at the multiconfigurational
level is certainly very appealing. Those developments, and others yet unforeseen, will
shape the future of the field and push the limits of what can be done, hopefully matching the significant advances in the experimental techniques. This can only improve
the already strong complementarity between theory and experiment and deepen our
insights into the captivating world of transition metal catalysis.
Acknowledgements We acknowledge financial support from the foundation Olle Engkvist Byggmastare and the Knut and Alice Wallenberg Foundation (Grant No. KAW-2013.0020). We thank
Meiyuan Guo and Michael Odelius for useful discussions.
References
1. Ågren H, Jensen HJA (1987) An efficient method for the calculation of generalized overlap
amplitudes for core photoelectron shake-up spectra. Chem Phys Lett 137(5):431–436
2. Ågren H, Flores-Riveros A, Jensen HJA (1989) An efficient method for calculating molecular
radiative intensities in the vuv and soft x-ray wavelength regions. Phys Scr 40(6):745
3. Andersson K, Malmqvist PÅ, Roos BO, Sadlej AJ, Wolinski K (1990) Second-order perturbation theory with a casscf reference function. J Phys Chem 94(14):5483–5488
4. Angeli C, Cimiraglia R, Evangelisti S, Leininger T, Malrieu JP (2001) Introduction of n-electron
valence states for multireference perturbation theory. J Chem Phys 114(23):10252–10264.
https://doi.org/10.1063/1.1361246
5. Aquilante F, Autschbach J, Carlson RK, Chibotaru LF, Delcey MG, De Vico L, Fdez Galván I,
Ferre N, Frutos LM, Gagliardi L et al (2016) Molcas 8: new capabilities for multiconfigurational
quantum chemical calculations across the periodic table. J Comput Chem 37(5):506–541
6. Atak K, Bokarev SI, Gotz M, Golnak R, Lange KM, Engel N, Dantz M, Suljoti E, Kühn O, Aziz
EF (2013) Nature of the chemical bond of aqueous Fe2+ probed by soft X-ray spectroscopies
and ab initio calculations. J Phys Chem B 117(41):12613–12618
M. Lundberg and M. G. Delcey
Yet, the X-ray modeling field is still evolving rapidly. This is certainly true for the
multiconfigurational approach, with new method developments constantly shaping
the way these calculations are performed. This process is likely to continue and
already now, many developments in related fields offer great promises to lift some of
the main limitations of the method. As an example, the CPP approach has been applied
to many wavefunction models to efficiently compute the spectrum at any energy
range [22]. While not available yet, an efficient CPP-CAS/RAS implementation
would alleviate the cost associated with the high density of states. It could also
provide higher accuracy by, for example, allowing core relaxation and ensuring
better consistency between different calculations by removing artifacts caused by
state averaging.
Similarly, while multiconfigurational simulations have mainly been limited to a
single metal atom because of active-space restrictions, many techniques have been
developed recently to push this limit, e.g., the density matrix renormalization group
[15], full CI quantum Monte Carlo [12], or Heat-Bath CI [37]. While those methods
still have not been used to compute X-ray spectra, and some technical difficulties are
still left to be overcome, the potential to calculate X-ray spectra of some of the fascinating natural and synthetic multi-metallic complexes at the multiconfigurational
level is certainly very appealing. Those developments, and others yet unforeseen, will
shape the future of the field and push the limits of what can be done, hopefully matching the significant advances in the experimental techniques. This can only improve
the already strong complementarity between theory and experiment and deepen our
insights into the captivating world of transition metal catalysis.
Acknowledgements We acknowledge financial support from the foundation Olle Engkvist Byggmastare and the Knut and Alice Wallenberg Foundation (Grant No. KAW-2013.0020). We thank
Meiyuan Guo and Michael Odelius for useful discussions.
References
1. Ågren H, Jensen HJA (1987) An efficient method for the calculation of generalized overlap
amplitudes for core photoelectron shake-up spectra. Chem Phys Lett 137(5):431–436
2. Ågren H, Flores-Riveros A, Jensen HJA (1989) An efficient method for calculating molecular
radiative intensities in the vuv and soft x-ray wavelength regions. Phys Scr 40(6):745
3. Andersson K, Malmqvist PÅ, Roos BO, Sadlej AJ, Wolinski K (1990) Second-order perturbation theory with a casscf reference function. J Phys Chem 94(14):5483–5488
4. Angeli C, Cimiraglia R, Evangelisti S, Leininger T, Malrieu JP (2001) Introduction of n-electron
valence states for multireference perturbation theory. J Chem Phys 114(23):10252–10264.
https://doi.org/10.1063/1.1361246
5. Aquilante F, Autschbach J, Carlson RK, Chibotaru LF, Delcey MG, De Vico L, Fdez Galván I,
Ferre N, Frutos LM, Gagliardi L et al (2016) Molcas 8: new capabilities for multiconfigurational
quantum chemical calculations across the periodic table. J Comput Chem 37(5):506–541
6. Atak K, Bokarev SI, Gotz M, Golnak R, Lange KM, Engel N, Dantz M, Suljoti E, Kühn O, Aziz
EF (2013) Nature of the chemical bond of aqueous Fe2+ probed by soft X-ray spectroscopies
and ab initio calculations. J Phys Chem B 117(41):12613–12618