Academic interests
Coupledcluster theory for periodic systems; the divideexpandconsolidate (DEC) algorithm.
Publications

Palmer, Teresa Lynne; Baardsen, Gustav & Skartlien, Roar (2018). Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations. Journal of Dispersion Science and Technology.
ISSN 01932691.
39(2), s 190 206 . doi:
10.1080/01932691.2017.1306784
Full text in Research Archive.

Hagen, Gaute; Papenbrock, Thomas; Ekström, Jan Andreas; Wendt, Kyle A.; Baardsen, Gustav; Gandolfi, Stefano; HjorthJensen, Morten & Horowitz, Charles J. (2014). Coupledcluster calculations of nucleonic matter. Physical Review C. Nuclear Physics.
ISSN 05562813.
89:014319 . doi:
10.1103/PhysRevC.89.014319
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Background: The equation of state (EoS) of nucleonic matter is central for the understanding of bulk nuclear properties, the physics of neutron star crusts, and the energy release in supernova explosions. Because nuclear matter exhibits a finely tuned saturation point, its EoS also constrains nuclear interactions. Purpose: This work presents coupledcluster calculations of infinite nucleonic matter using modern interactions from chiral effective field theory (EFT). It assesses the role of correlations beyond particleparticle and holehole ladders, and the role of threenucleon forces (3NFs) in nuclear matter calculations with chiral interactions. Methods: This work employs the optimized nucleonnucleon (NN) potential NNLOopt at nexttonextto leading order, and presents coupledcluster computations of the EoS for symmetric nuclear matter and neutron matter. The coupledcluster method employs up to selected triples clusters and the singleparticle space consists of a momentumspace lattice. We compare our results with benchmark calculations and control finitesize effects and shell oscillations via twistaveraged boundary conditions. Results: We provide several benchmarks to validate the formalism and show that our results exhibit a good convergence toward the thermodynamic limit. Our calculations agree well with recent coupledcluster results based on a partial wave expansion and particleparticle and holehole ladders. For neutron matter at low densities, and for simple potential models, our calculations agree with results from quantum Monte Carlo computations. While neutron matter with interactions from chiral EFT is perturbative, symmetric nuclear matter requires nonperturbative approaches. Correlations beyond the standard particleparticle ladder approximation yield nonnegligible contributions. The saturation point of symmetric nuclear matter is sensitive to the employed 3NFs and the employed regularization scheme. 3NFs with nonlocal cutoffs exhibit a considerably improved convergence than their local cousins. We are unable to find values for the parameters of the shortrange part of the local 3NF that simultaneously yield acceptable values for the saturation point in symmetric nuclear matter and the binding energies of light nuclei.

Baardsen, Gustav; Ekström, Jan Andreas; Hagen, Gaute & HjorthJensen, Morten (2013). Coupledcluster studies of infinite nuclear matter. Physical Review C. Nuclear Physics.
ISSN 05562813.
88(5) . doi:
10.1103/PhysRevC.88.054312
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Background: Coupledcluster (CC) theory is a widely used manybody method for studying strongly correlated manyfermion systems. It allows for systematic inclusions of complicated manybody correlations beyond a mean field. Recent applications to finite nuclei have shown that firstprinciples approaches like CC theory can be extended to studies of mediumheavy nuclei, with excellent agreement with experiment. However, CC calculations of properties of infinite nuclear matter are rather few and date back more than 30 yr. Purpose: The aim of this work is thus to develop the relevant formalism for performing CC calculations in nuclear matter and neutronstar matter, including thereby important correlations to infinite order in the interaction and testing modern nuclear forces based on chiral effective field theory. Our formalism includes also the exact treatment of the socalled Pauli operator in a partialwave expansion of the equation of state. Methods: Nuclear and neutronmatter calculations are done using a coupled particleparticle and holehole ladder approximation. The coupled ladder equations are derived as an approximation of CC theory, leaving out particlehole and nonlinear diagrams from the CC doubles amplitude equation. This study is a first step toward CC calculations for nuclear and neutron matter. Results: We present results for both symmetric nuclear matter and pure neutron matter employing stateoftheart nucleonnucleon interactions based on chiral effective field theory. We employ also the newly optimized chiral interaction [ Ekström et al. Phys. Rev. Lett. 110 192502 (2013)] to study infinite nuclear matter. The ladder approximation method and corresponding results are compared with conventional BruecknerHartreeFock theory. The ladder approximation is derived and studied using both exact and angularaveraged Pauli exclusion operators, with angularaveraged input momenta for the singleparticle potentials in all calculations. The inclusion of an exact treatment of the Pauli operators in a partialwave expansion yields corrections of the order of 1.7%–2% of the total energy in symmetric nuclear matter. Similarly, the inclusion of both holehole and particleparticle ladders result in corrections of the order 0.7%–2% compared to the approximation with only particleparticle ladders. Including these effects, we get at most almost a 6% difference between our CC calculation and the standard BruecknerHartreeFock approach. Conclusions: We have performed CC calculations of symmetric nuclear matter and pure neutron matter including particleparticle and holehole diagrams to infinite order using an exact Pauli operator and angularaveraged singleparticle energies. The contributions from holehole diagrams and exact Pauli operators add important changes to the final energies per particle.

Ekström, Jan Andreas; Baardsen, Gustav; Forssen, C; Hagen, Gaute; HjorthJensen, Morten; Jansen, Gustav R.; Machleidt, Ruprecht; Nazarewicz, witek; Papenbrock, Thomas; Sarich, Jason & Wild, Stefan (2013). Optimized Chiral NucleonNucleon Interaction at NexttoNexttoLeading Order. Physical Review Letters.
ISSN 00319007.
110(19) . doi:
10.1103/PhysRevLett.110.192502
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We optimize the nucleonnucleon interaction from chiral effective field theory at nexttonexttoleading order (NNLO). The resulting new chiral force NNLOopt yields χ2≈1 per degree of freedom for laboratory energies below approximately 125 MeV. In the A=3, 4 nucleon systems, the contributions of threenucleon forces are smaller than for previous parametrizations of chiral interactions. We use NNLOopt to study properties of key nuclei and neutron matter, and we demonstrate that many aspects of nuclear structure can be understood in terms of this nucleonnucleon interaction, without explicitly invoking threenucleon forces.
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Rebolini, Elisa; Baardsen, Gustav; Hansen, Audun Skau; Leikanger, Karl Roald & Pedersen, Thomas Bondo (2017). Local coupled cluster methods for periodic systems.

Leikanger, Karl R.; Rebolini, Elisa; Hansen, Audun Skau; Baardsen, Gustav & Pedersen, Thomas Bondo (2016). HartreeFock calculations using a priori Wannier orbitals for solids.

Baardsen, Gustav (2014). Coupledcluster theory for infinite matter.
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Published Jan. 13, 2016 10:23 AM
 Last modified Feb. 10, 2016 8:37 AM