Posted on September 8, 2014 at 7:05 PM |

My talk '*New Stringent Conditions for the Cumulant Matrix*' was selected for 10th edition of the World Association of Theoretically-Oriented Chemists (WATOC) that will be celebrated in Santiago de Chile. In addition, I was invited to give a talk in the satellite meeting "New Approaches in Theoretical Chemistry" that will be held in the University of Chile in Santiago two days before WATOC.

Posted on July 19, 2014 at 6:55 PM |

Last week the European Young Chemist Award Organization notified me that I had been selected among the finalists of the EYCA2014. The final step of this competition is a 10min talk that takes places at the 5th European Chemistry Congress that I will attend in Istanbul. As far as I remember there are eight candidates in the postdoc category; one is chosen for the gold medal and two for the silver one. Needless to say, I'm very happy with the nomination and I will do my best at Istanbul. :-)

Posted on June 29, 2014 at 1:00 PM |

This summer the group will enjoy the visit of a few researchers. Some of them will come next week to attend the XI edition of the Girona Seminar and the others will arrive during the following weeks and stay for a longer time.

The Girona Seminar will bring a few collaborators: Dr. J. Óscar C. Jiménez-Halla (alumni currently in Univ. Guanajuato), Prof. Jesus Ugalde (Univ. Basque Country), Prof. Gabriel Merino (Univ. Mérida) and Prof. Patrick Bultinck (Univ. Ghent). The visit of Óscar will help to push forward a current research project about borole aromaticity with Prof. Braunschweig. Patrick's student, Jelle Vekeman, whose master thesis I have co-promoted, will defend his thesis after the closure of the Girona Seminar.

Next week, Prof. Jacek Styszynsky will stay in Girona and give a talk about our recently published work on metal carbonyl complexes. The same week, Sebastian Sitkiewicz will join the group for a month stay. Sebastian is an Erasmus student and he will be working on the developmet of DFT functionals. Hopefully, Sebastian's visit will be the first of many others visits to our group. The fourth week of July, Dr. Tomás Rocha (an old friend from my postdoctoral time in Denmark) will be paying us a visit. Tomás will stay for two weeks and we will start a collaboration project together.

Posted on June 18, 2014 at 6:45 PM |

In the last years we have worked in the design of a new approximation to the third-order density that just got accepted for publication in J. Chem. Theory Comput. The nth order density is a well-known quantity in quantum mechanics that gives the probability of finding n particles (usually electrons) simultaneously in different location of the molecular space. The first and second-order densities are actually needed for the calculation of the electronic energy of a given molecular system. In principle, higher-order densities do not enter the energy expression, however, the calculations performed with the contracted Schrödinger Equation and its antiHermitian version do use three and four-order density. In addition, the probability of finding three and four electrons depends on these densities. The latter is main focus of our development, since we want to use these quantities to calculate three and four-center indices. In particular, in this paper we study systems with three-center bonds, such as diborane, which contains a three-center two-electron bond.

The approximations we suggest for the third-order densities can be casted in the following expression:

where gamma is the pure three-body part and depends on a parameter *a*:

This expression can be used with *a*=1 (HF-like approach) and *a*=1/2 and *a*=1/3, which are the approximations suggested in this work. The calculation on a series of molecules proves that a=1/3 works significantly better than other approximations to the third-order density such as Valdemoro's, Mazziotti's or Nakatsuji's. We are currently trying to understand why our approximations works so well and see if its applicability can be extended to other properties.

Posted on June 12, 2014 at 4:25 AM |

This year research will take me to travel around the world. Literally :-) My trip starts on the 8th of August and I will be back in Girona the 7th of September. I will attend three conferences and visit a research group.

My journey starts in San Franciso. I will participate in the 248th ACS meeting in San Francisco where I will give two oral presentations:

- August 12, 3:45 pm. Division of Computers in Chemistry. Quantum Chemistry Session.

"*Multicenter Bond Index: A versatile tool to characterize electron delocalization and aromaticity*".

- August 14, 3:45 pm. Division of Computers in Chemistry. Quantum Chemistry Session.

“New stringent conditions for the cumulant matrix”.

The first talk summarizes our research on multicenter bond indices with Ferran Feixas and Miquel Solà: from the first works during Ferran's thesis, until our last papers on applications of these indices aimed to characterize agostic interactions and approximate multicenter indices. The second talk focuses on two new stringent conditions for the two-particle cumulant matrix. This work has been carried out with Eloy Ramos, Pedro Salvador and Mario Piris, and introduces two stringent conditions that the most sophisticated natural orbital functionals fail to attain. Interestingly, these conditions do not only reveal important limitations of current functionals but also suggest modifications to improve them.

After the ACS meeting I will fly to Chicago to visit Prof. David Mazziotti's group. Mazziotti is a renown specialist in the field of density matrices with whom I would like to discuss about our recent approximation for a third-order reduced density matrix. I will give a talk on Mazziotti's covering this topic. Our approximation works better than any other for the calculation of n-variate probabilities, but we have not used it yet in the calculation of electronic energies. It would be interesting to see whether our approximation can be used in this context as well.

My third stop takes me to Vietnam. The Current Topics in Theoretical Chemistry workshop organized by Julia Contreras-Garcia, Andreas Savin and Ramon Carbó takes places in the city of Nha Trang. I have been invited to give a 45min presentation on Aromaticity. This lecture is a rather introductory talk on the subject, which also covers a few glimpses of my research on the topic over the last years. This workshop includes other interesting topics, including method development lectures by well-known experts such as John Perdew, Trygve Helgaker, Gustavo Scuseria, Jeppe Olsen or Weitao Yang, among many others. It will be a great opportunity to make new scientific connections and meet some current collaborators.

The last station is Turkey. I have applied for a talk in the 5th EuCheMS conference held in Istanbul. This conference is less theoretically oriented and, if chosen for an oral presentation, I will present a recent work on a new kind of materials known as electrides.

Altogether it will be a month tour around the world including four to five lectures, the opportunity to meet many researchers and, hopefully, establishing new fruitful collaborations.

**Update 29/06/2014**. My talk "Characterization and Identification of Electrides" has been chosen for an oral presentation in the 5th EuCheMS conference.

Posted on May 10, 2014 at 9:45 AM |

This year, a work published in Chem. Eur. J. was chosen for a cover in this journal. This work is devoted to Inverse carbon-free sandwich structures with formula E2P4 (E=Al, Ga, In, Tl), which have been proposed as a promising new target in main-group chemistry. Our computational exploration of their corresponding potential-energy surfaces at the S12h/TZ2P level shows that indeed stable carbon-free inverse-sandwiches can be obtained if one chooses an appropriate Group 13 element for E. The boron analogue B2P4 does not form the D4h-symmetric inverse-sandwich structure, but instead prefers a D2d structure of two perpendicular BP2 units with the formation of a double B[BOND]B bond. For the other elements of Group 13, Al–Tl, the most favorable isomer is the D4h inverse-sandwich structure. The preference for the D2d isomer for B2P4 and D4h for their heavier analogues has been rationalized in terms of an isomerization-energy decomposition analysis, and further corroborated by determination of aromaticity of these species.

Posted on April 24, 2014 at 8:10 AM |

On March we meet with the working team for a group photo. The group is slowly growing and now it includes one PhD student, one full-time postdoctoral fellow and partial-time one as well as three undergraduate students. They have all been employed by the Europa Excelencia project from MINECO.

From left to right: Mr. Mauricio Rodríguez, Ms. Natàlia Abulí, Ms. Irene Casademont, Dr. Eloy Ramos-Córdoba, Mr. Jesus Antonio Luque, Dr. Eduard Matito and Dr. Ferran Feixas.

Mauricio is currently on the second year of his PhD thesis and works on the development of DFT functionals with the use of model systems. Natàlia is a last-year undergradute student working in the development of DFT functionals for the calculation of non-linear optical properties. Irene is a third-year undergraduate student working on Dye Sensitized Solar Cells using time-dependent DFT and several tools developed in the group to study the charge-transfer process. Eloy is a postdoctoral fellow working on method development, including DFT, DMFT, electron-correlation measures and local spin analysis. Jesus A. is a third-year undergraduate working on chemical bonding analysis with focus on multicenter interactions. Ferran is a postdoctoral fellow focused on protein dynamics (his own research line) and working with us on the construction of approximate high-order density matrices and chemical bonding analysis.

Posted on February 3, 2014 at 9:40 AM |

2013 has been a great year. I started the year knowing that my project DFTCorr was selected for the final stage of the Starting Grant (StG). I had the interview in Brussels on April. Unfortunately, it did not went as good as I would like to; one of the members of the committee was clearly against my proposal. On June I got the outcome. It was very good (A) but it was not enough to be granted the StG. I was on the 53%-55% ranking range (the 1-50% range was awarded). Since then, I started working on the Consolidator Grant 2014 proposal, which I will submit at the end of May 2014.

On May 2013, I got the outcome of the Ramon y Cajal grant (Spanish five-year tenure track). It did not matter that I have passed to the final stage of the StG... this time they did not even rank me among the first fifty. I gave up on this proposal, is too much biased towards high-IF research. My research does not get published in JACS or Angewandte. In my humble opinion it is completely unfair to those of us who work on more theoretically-oriented research, but I cannot help it. I better focus on non-Spanish calls, which are less driven by this IF craziness.

At the end of the year I got two good news. On one side, I was one of the eight researchers awarded a Europa Excelencia project (€75,000). Only those researchers that submitted a StG in the last call (2013), passed to the last stage in Brussels and get the maximum score (A), could apply. IMHO it is a bit unfair that other researchers who also made it to the final stage in Brussels but got a B could not apply for these projects. I'm using this money to purchase a small workstation and hire several people to work on the DFTCorr project.

On December I learnt I was awarded a five-year position at the Basque Country with possibility of permanent position after evaluation on the fourth year. The position comes also with some startup money and a travel allowance from the Ikerbasque foundation [total funding: €215,408].

Research-wise it was also a very good year. Several important results have been obtained, including a very precise approximation for the third-order reduced density matrix (3-RDM) [work submitted], a new stringent condition for the density matrix functional theory (DMFT) [work to be submitted soon] -that in principle can be extended to DFT- and several partial results on the DFT project that can be used to reinforce the CoG application of 2014. I just need the time to write down all this stuff... :-) Hopefully I can start before the end of May!

Addendum 1 (March 2014): Several newspapers and websites published the news about the Europa Excelencia and the Ikerbasque position. Local News: UdG, Science & Technology Park. Catalan Newspapers: Punt Avui, Diari de Girona and La Vanguardia. Spanish Newspaper: ABC.

Addendum 2 (April 2014): My PhD student, Mauricio Rodriguez, who is working on the DFTcorr project, was awarded the Master Thesis award and obtained a national PhD grant. On the other hand, Eloy Ramos obtained his PhD diploma, and he will be soon hired as postdoc to continue his work on DFTcorr.

Addendum 3 (May 2014): Verònica Postils was awarded the Sant Jordi award for his work '*Electrides. Tomorrow's compounds*', based on her Master Thesis (September 2013) that I co-supervised.

Posted on January 4, 2014 at 6:40 PM |

Anyone working on research knows that very seldom one has the chance to start and finish a paper within a short period of time. Usually a research project extends over a long period, with some lapses of time when the project gets stuck or halt for this or that reason. Namely, projects in collaboration with people from different institutions tend to be slow running projects. The story I explain in this post completely feels in this category. The research started on November 2004 and it got published on September 2013.

It all started back in 2004, when I was a PhD student visiting for the first time the University of Szczecin (Poland). I went to Szczecin to work with Jerzy Cioslowski, who agreed on hosting me for three months. I had read some paper of Jerzy and I was enthusiastic to have the chance to work with him. So, I got quite disappointed when I realized that he would be on a short stay in Dresden for most of my time in Szczecin. At least, he arranged someone to look after me. He was Jacek Styszynski, a physicist working on relativistic effects on molecules. Jacek took good care of me and had the patience to teach me about (relativistic) four-components calculations. Under his guidance in no more than three months we completed a research project that we published in Chemical Physics. This is the counterexample of how much time a collaborative project takes. Afterwards, Jacek suggested to work on a more ambitious project, which pretended to produce benchmark results for several carbonyl complexes and explain some controversial results obtained in the literature. The work was very appealing from both the chemical and the computational viewpoints. By then, there were very few relativistic codes and none of them could perform geometry optimization. So we would have to implement the optimization and the frequency calculations of these species. In 2004 I started working on this project for a few weeks before Jerzy came back. Then, I switched to some research project about the homogeneous electron gas. Short after Jerzy returned I went back to Girona and basically left Jacek's work aside for a while.

In 2005 I visited again Szczecin. This time I worked on both projects full time, and we got some results on the project with Jacek. However, at the end of my stay, after careful check of our results we noticed a large deviation of some vibrational frequencies as compared to the experimental ones. This result made no sense and it was Jerzy who noticed that our scheme to calculate vibrational frequencies (taken from an old German manuscript) missed some important coupling effects. On Christmas 2005 I left Szczecin with the feeling we still had plenty of work to do.

On July 2006 I defended my thesis, and right after that I went to Szczecin for another four-month stay. This time the focus of my stay in Szczecin was completing with Jerzy some paper on aromaticity and preparing the postdoc project I would carry out in Szczecin. I also spent quite some time working on Jacek's project but I left Szczecin with the feeling that the metal carbonyl project needed some re-thinking and we were far from the end...

On 2006 I was not awarded two (Spanish and Catalan) postdoctorals to go to Szczecin, so I look for another postdoc possibility. That is how I ended up in Aarhus, working with Ove Christiansen. I had a great time in Denmark and learnt a lot about vibrational calculations, potential energy surfaces, coupled-cluster methods and even some QM/MM theory. If you want to learn these techniques, visiting Aarhus (and Ove) is a must. At the beginning of 2007 I was awarded the Marie Curie fellowship to work in Szczecin but I enjoyed so much my time in Aarhus that I postponed my start in Szczecin more than one year. (I wish it could have been more!) Nonetheless, I started in Szczecin full of enthusiasm, because I was giving the chance to start doing some research on model systems, and I needed this background to later work on theory development as I really wanted to do.

Altogether, I had abandoned my project with Jacek two years and I took it back ion Spring 2009. After my experience in Aarhus I knew how to deal with the vibrational problem of metal carbonyls. Since I was mostly focused on the research about harmonium it took one year to obtain the first optimization results from Dirac (which was now capable of geometry optimization) and realize that the optimizations were neither efficient nor reliable. This finding coincided with Pedro Salvador visit to Szczecin, and he got enrolled in the project. Using a modified code of his, we interfaced a modified version of Dirac to obtain more reliable geometry optimizations and harmonic frequencies. Some months later we obtained our first final results. So, at the end of my postdoctoral stay in Szczecin we had most of our geometries and frequencies for the lightest transition metals.

On 2011 I moved to the Basque Country, where I had basically no time to work on Jacek's project. On 2012 I moved back to Girona but, once again, it took me a few months to find the time to work on the metal carbonyl work. By then, Jacek had completed most calculations. I started writing the manuscript at the beginning of 2013 and generated the chemical bonding analysis of these complexes. On Spring we submitted our manuscript, which got finally accepted on Autumn. It got published before the end of 2013.

This project took longer than expected because it was never part of my main research subject but also because of the research itself. If you are familiar with computational chemistry you may wonder how a project of three-atom molecules can take so long. The reason is four-component calculations. Unlike (regular) two-component methods, there are very few basis sets (sometime none!) available for these complexes. We had to change the basis sets when better ones were available (Prof. Dyall kindly sent us these basis set in advance). On top of this, one should take into account that there was no gradients available in Dirac for four-components CCSD(T) and, therefore, the optimization and frequency calculations are done numerically with the aid of an external software. The nature of the potential energy surface of MCO molecules also complicated both the geometry optimization and the frequency calculations. Our results included forth-row transition metals without the use of pseupotentials (they are not available); these were by far the most expensive calculations. Hopefully, we proved that Sadley's X2C methods, which are much more less demanding than pure four-component methods, give good results for geometries, dissociation energies and CO stretching frequencies (although a few bending frequencies are wrongly estimated with this method).

I am happy that despite the difficulties we could complete the paper with Jacek and Pedro. The metal carbonyl project was a good excuse to keep in touch with Jacek, who was my neighbor in Szczecin for two years and he is now also a very good friend. Jadwiga and him made my time in Szczecin a great and fruitful experience. If we ever get enrolled in a research project together with him let us hope it will take far less time. My feeling is that I will have the opportunity to see it by myself :-)

Posted on October 28, 2013 at 10:20 PM |

This year I have attended four different conferences and I have presented a few works, including some results for aromaticity, multicenter indices, third-order densities, electrides and DFT development.

On June I participated in the NCI Workshop *Topological Approaches to Intermolecular Interactions* organized, among others, by Julia Contreras from the CNRS in Paris. The conference included a training session, in which the authors and developers of NCI software gave us a good introduction to the theory and the software. It was a very well organized conference which included four talks from people from the IQCC (Verònica Postils, Marc Garcia-Borràs, Sílvia Simon and myself) and one excellent presentation by Ferran Feixas, a former IQCC member. I presented a summary of the results we have obtained in the past years using the multicenter indices. In addition, I got the chance to perform as a session chairman.

On July I was invited to give the course *Molecular vibrations: From Vibrational Wavefunctions to Potential Energy Surfaces* in the 9th SEADIM Intensive Course organized in Havana (Cuba). As a curiosity I got a fever the day of the lecture and I did my best to give a good talk... but I can only certainly say that this is the lecture in which I have sweated the most :-) A few days later I took part in the 9th Seminars of Advanced Studies on Molecular Design and Bioinformatics: Energy (SEADIM) organized in Varadero, giving a talk entitled *A New Means to Characterize and Identify of Electrides. *

On September I attended the 15th DFT conference held in Durham (UK) which, so far, has been the most fruitful conference of the year. I brought a poster presenting new hybrids functionals for two and three-electron harmonium. In Durham I enjoyed very good lectures, including those of Axel Becke, Erin Johnson and Marcel Swart. The last one was particularly enlightening and, IMHO, the best conference of Marcel I have attended (Marcel is also part of the IQCC).

Finally, on October I was invited to give a lecture in *Advances in Quantum Chemical Topology*, a very well-organized symposium that took place in Mexico City. My talk summarized the last results obtained with multicenter indices but with particular focus on the new approximation for the third-order density we have designed (work to be submitted soon).

And basically that's it for 2013. In a forthcoming post I shall give more details of my stay in Mexico.