RESEARCH

EXPERTISE
• Ground state properties of solids and molecules: equilibrium geometry and electronic structure in the DFT framework with plane wave and localized basis sets. Use of public codes: Abinit, PWscf, Octopus, Gaussian, NWChem

• Electronic band structure calculations of bulk and quasi-one dimensional systems (polymers) within DFT and GW methods. Use of public codes: Abinit, PWscf, Yambo.

• Optical properties of nanostructures within Time Dependent Density Functional Theory in time and frequency domain and with the inclusion of excitonic effects. Use of public codes: Yambo, Octopus

• Quantum Montecarlo Methods for fermions with variational and diffusion algorithms. (inhouse code)

CURRENT SCIENTIFIC ACTIVITY AND INTEREST
My main activities are concerned with the theoretical and computational study of electronic and optical properties of low dimensional systems with ab-initio techniques. The main research topics are:

a) Optical properties of biological photo-receptors. My activity in this field is devoted to the study of bio-physical processes in the microscopic description of the photo-response of bio-molecules. Within the Time Dependent Density Functional Theory framework we have calculated the optical response of the Green Fluorescent Protein (GFP), of the DNA bases and their assemblies, and of modified aromatic-expanded DNA bases. The computed absorption spectrum of GFP is in close agreement with the measured spectrum and gives support to the suggested model of the GFP photo-physics that contains two thermodynamically stable neutral and anionic configurations of the cromophore. With the same theoretical technique we have studied the absorption spectra of isolated DNA bases. The results are in agreement with experiment and previous calculations. Then I have inspected the impact on the optical properties of the H-bonding when the bases are paired in theWatson-Crick configuration, and of the stacking interactions between base pairs. Currently I am studying the optical properties of the bases and assemblies of bases of a modified aromatic-expanded DNA (xDNA), a modified genetic system that is a good candidate for molecular electronic due to the enhanced stacking interactions between planes of base pairs. Moreover, I am co-developing a theory that permits the calculation of circular dichroism in the TDDFT framework.
This research activity has started during my Phd studies and is now going on during my Post-doc, in collaboration with Dr. Rosa Di Felice, Prof. Angel Rubio and Prof. Miguel Marques.

b) Electronic properties of G4-DNA with sequence alternations explored for devices. My research activity in this field is devoted to the theoretical study of DNA-derivatives in order to implement DNA-based electrical devices. The electrical conductivity of native-DNA is still disputable, and a possible alternative to go beyond the limits of native-DNA is to design and realize modifications of the DNA structure, such as an alteration of the helical motif from the doublestranded helix to a quadruple helix. It has been demonstrated that the electronic structure of a quadruple helix made of guanines reveals better channels for charge motion than in native DNA. I am now investigating if an alternation of the sequence, obtained by inserting adenine tetrads into the stack of guanine tetrads , may interrupt the electronic channel and behave as an electric non-linear element.
This work is done in collaboration with Dr. Rosa Di Felice. In addition, I am investigating the electronic coupling matrix elements, between adjacent base pairs in xDNA (see above).

c) Optical properties of molecular chains and quasi-one dimensional systems. One dimensional molecular chains (i.e. polymers) are paradigmatic systems where independent particle approximations fail to furnish a good description of the optical properties, or the static polarizability. During my PhD I have studied such systems showing that the spatial confinement plays an important role in enhancing the short distance interactions. Due to this fact an appropriate description of the optical properties of such systems, as a function of the chain length, can be obtained considering the electron-hole attraction in a many-body approach, based on the Green’s function method (Bethe Salpeter equation). Moreover I have shown that the saturation of the polarizability is driven by exciton localization and that the TDDFT approach can be also used when sophisticated kernels extracted from the many-body perturbation theory are used.
This work has been done in collaboration with Dr. Andrea Marini and Prof. Angel Rubio.
Actually I am studying, within the Bethe Salpeter framework the excitonic effects present in the absorption spectrum of graphene nano-ribbons, a novel quasi-one-dimensional carbon-based systems, suitable for future nanoscale optoelectronics applications.
This work is done in collaboration with Deborah Prezzi, Dr. Alice Ruini and Prof. Elisa Molinari.

PAST RESEARCH EXPERIENCE
During my research period at the University of Romw ”La Sapienza” my research activity has been focused on the study of the magnetic phase of the 2D electron gas near freezing. This work has be done in collaboration with Dr. Saverio Moroni, and Prof. Gaetano Senatore. We performed variational and diffusion Monte Carlo calculation in the fixed-node approximation. We found the evidence for a polarization transition at densities achievable nowadays in 2-dimensional hole gases in semiconductor heterostructures. The spin susceptibility of the unpolarized phase at themagnetic transition is approximately 30 times the Pauli susceptibility. Moreover, at variance with the 3D case we found no evidence for the stability of a partially polarized phase.

CODE DEVELOPMENT
Since 2002 I am involved in the developers team of the high performance computing parallel code Yambo: (www.yambo-code.org). Yambo is a FORTRAN/C code for Many-Body calculations in solid state physics. It relies on the Kohn-Sham wave-functions generated by several DFT public codes as Abinit (www.abinit.org) and PWscf (www.pwscf.org). It uses plane wave basis sets, and permits the calculation of photo-emission spectra within the GW approximation and optical properties via Time Dependent Density Functional Theory and it includes excitonic effects via the Bethe Salpeter equation. My main contributions in the code development have been: i) the development and implementation of a technique to permit the treatment of finite and quasi-one dimensional systemswithin the supercell approximation, ii) the implementation of the Bethe-Salpeter equation beyond the Tamm-Dancoff approximation and iii) the development of the excitonic wavefunctions drawing tools.

LIST OF PUBLICATIONS

INTERNATIONAL JOURNALS

1. A. Calzolari, D. Varsano, A. Ruini, A. Catellani, R. Tel-Vered, H.B. Yildiz, O. Ovits, Oded; I. Willner, "Optoelectronic properties of natural cyanin dyes", J. Phys. Chem. A 113 8801 (2009).
2. L. Nielsen, A. Holm, D. Varsano, U. Kadhane, S. Hoffmann, R Di Felice, A. Rubio and S. Nielsen "Fingerprints of bonding motifs in DNA duplexes of adenine and thymine revealed from circular dichroism: synchrotron radiation experiments and TDDFT calculations", J. Phys. Chem B 113, 9614 (2009).
3. A. Migliore, S. Corni, D. Varsano, M. L. Klein, and R. Di Felice "First-principles effective electronic couplings for hole transfer in natural and size-expanded DNA", J. Phys. Chem. B 113, 9402 (2009).
4. D. Varsano, L. A. Espinosa-Leal, X. Andrade, M. A. L. Marques, R. Di Felice, and A. Rubio, "A gauge invariant method for molecular chiroptical properties in TDDFT", Phys. Chem. Chem. Phys. 11, 4481 (2009).
5. N. Spallanzani, C. A. Rozzi, D. Varsano, T. Baruah, M. R. Pederson,F. Manghi, and A. Rubio, "Photo-excitation of a light-harvesting supra-molecular triad: a Time-Dependent DFT study", J. Phys. Chem. B 113, 5345 (2009).
6. A. Marini, C. Hogan, M. Gruning and D. Varsano, "yambo: an ab-initio tool for excited state calculations" Comp. Phys. Comm., in press (2009).
7. A. Castro, M. A. L. Marques, D. Varsano, F. Sottile and A. Rubio "The challenge of predicting optical properties of biomolecules: What can we learn from time-dependent density-functional theory", Compte Rendus Physique, in press (2008).
8. D. Varsano, A. Marini and A. Rubio, "Optical saturation driven by exciton confinement in molecular-chains: A time-dependent densityfunctional theory approach", Phys. Rev. Lett. 101, 133002 (2008).
9. D. Prezzi, D. Varsano, A. Ruini, A. Marini, E. Molinari, "Optical properties of graphene nanoribbons: The role of many-body effects" Phys. Rev. B 77, R041404 (2008).
10. D. Prezzi, D. Varsano, A. Ruini, A. Marini, E. Molinari, "Optical properties of one-dimensional graphene polymers: the case of polyphenanthrene", phys. stat. sol. (b) 244, 4124 (2007).
11. D. Varsano, A. Garbesi and R. Di Felice, "Ab initio optical absorption spectra of size-expanded xDNA base assemblies", J. Phys. Chem. B 111, 14012 (2007).
12. C. A. Rozzi, D. Varsano, A. Marini, A. Rubio and E.K.U Gross, "An exact Coulomb cutoff technique for supercell calculations" Phys. Rev. B 73, 205119 (2006).
13. D. Varsano, R. Di Felice, M.A.L Marques and A. Rubio, " A TDDFT study of excited states of DNA bases and their assemblies:, J. Phys. Chem. B 110, 7129 (2006).
14. D. Varsano, M.A.L. Marques and A. Rubio, "Time and Energy resolved two photon photoemission of the Cu(100) and Cu(111) metal surfaces", Computational Material Science 30, 110 (2004).
15. M.A.L. Marques, X. Lopez, D.Varsano, A. Castro and A. Rubio, "Time Dependent Density Functional approach for biological chromophore: The case of the Green Fluorescent Protein", Phys. Rev. Lett. 90, 258101 (2003).
16. D. Varsano, S. Moroni and G. Senatore, "Spin-polarization transition in the two-dimensional electron gas", Europhys. Lett. 53 348 (2001).
17. G. Senatore, S. Moroni and D. Varsano, "Spin effects in the 2D electron gas", Sol. St. Commun. 119, 333 (2001).
18. G. Senatore, S. Moroni and D. Varsano, "Spontaneous magnetization of the 2D electron gas", Computer Phys. Commun. 142, 406 (2001).

CHAPTER OF BOOKS

1. R. Di Felice, A. Calzolari, D. Varsano and A. Rubio, "Electronic structure calculations for nanomolecular systems in Introducing Molecular Electronics, edited by G.Cuniberti. K. Richter and G. Fargad. Lecture Notes in Physics Vol.680 (Springer, Berlin, 2005). Proceedings.
2. A. Castro, M.A.L Marques, X. Lopez, D. Varsano and A. Rubio, "Excited states properties of nanostructures and biomolecules through time dependent density functional theory", Proceedings of the 3rd International Conference ”ComputationalModeling and Simulation of Materials&Special Symposium” Modeling and SimulatingMaterialsNanoworld”, Acireale, Italy, 2004.

POPULAR SCIENCE ARTICLES

1. D. Varsano, "Role of H-bonding and ¼ stacking in the optical absorption of DNA nucleobase assemblies", Bulletin of the Network of Excellence Nanoquanta, 28 October 2005.
2. M.A.L Marques, X. Lopez, D. Varsano, A. Castro and A. Rubio, "Understanding biological photoreceptors", Donostia International Physics Center Highlight, 2003.
3. A. Rubio, D. Varsano, M. Marques, A. Castro, J. Serrano, M. J. Lopez, V. M. Silkin and P.M. Echenique, "Simulaci´o de les propietats electr´oniques i estructurals de nanotubs i nanoestructures i les seves aplicacions tecnol´ogiques", TERAFLOP Revista del centre de supercomutaci ´o de Catalunya 61, 21, (2001).