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<article article-type="research-article" dtd-version="1.3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:lang="en"><front><journal-meta><journal-id journal-id-type="publisher-id">mrisel</journal-id><journal-title-group><journal-title xml:lang="en">Magnetic Resonance in Solids</journal-title><trans-title-group xml:lang="ru"><trans-title>Magnetic Resonance in Solids</trans-title></trans-title-group></journal-title-group><issn pub-type="epub">2072-5981</issn><publisher><publisher-name>Kazan Federal University</publisher-name></publisher></journal-meta><article-meta><article-id custom-type="elpub" pub-id-type="custom">mrisel-222</article-id><article-categories><subj-group subj-group-type="heading"><subject>Research Article</subject></subj-group><subj-group subj-group-type="section-heading" xml:lang="ru"><subject>Статьи</subject></subj-group></article-categories><title-group><article-title>Feynman-Kac path integrals and excited states of quantum systems (in English)</article-title><trans-title-group xml:lang="ru"><trans-title></trans-title></trans-title-group></title-group><contrib-group><contrib contrib-type="author" corresp="yes"><name-alternatives><name name-style="western" xml:lang="en"><surname>Fazleev</surname><given-names>N. G.</given-names></name></name-alternatives><bio xml:lang="en"><p>Arlington, Texas 76019-0059; Kazan, 420008, Russia</p></bio><xref ref-type="aff" rid="aff-1"/></contrib><contrib contrib-type="author" corresp="yes"><name-alternatives><name name-style="western" xml:lang="en"><surname>Fry</surname><given-names>J. L.</given-names></name></name-alternatives><bio xml:lang="en"><p>Arlington, Texas 76019-0059</p></bio><xref ref-type="aff" rid="aff-2"/></contrib><contrib contrib-type="author" corresp="yes"><name-alternatives><name name-style="western" xml:lang="en"><surname>Rejcek</surname><given-names>J. M.</given-names></name></name-alternatives><bio xml:lang="en"><p>Arlington, Texas 76019-0059</p></bio><xref ref-type="aff" rid="aff-2"/></contrib></contrib-group><aff xml:lang="en" id="aff-1"><institution>Department of Physics, Box 19059, University of Texas at Arlington; Department of Physics, Kazan State University</institution><country>United States</country></aff><aff xml:lang="en" id="aff-2"><institution>Department of Physics, Box 19059, University of Texas at Arlington</institution><country>United States</country></aff><pub-date pub-type="collection"><year>2004</year></pub-date><pub-date pub-type="epub"><day>22</day><month>01</month><year>2024</year></pub-date><volume>6</volume><issue>1</issue><issue-title>Special Issue</issue-title><fpage>37</fpage><lpage>49</lpage><permissions><copyright-statement>Copyright &amp;#x00A9; Fazleev N.G., Fry J.L., Rejcek J.M., 2024</copyright-statement><copyright-year>2024</copyright-year><copyright-holder xml:lang="ru">Fazleev N.G., Fry J.L., Rejcek J.M.</copyright-holder><copyright-holder xml:lang="en">Fazleev N.G., Fry J.L., Rejcek J.M.</copyright-holder><license xml:lang="ru" license-type="creative-commons-attribution" xlink:href="https://creativecommons.org/licenses/by/4.0/" xlink:type="simple"><license-p>Данная работа распространяется под лицензией Creative Commons Attribution 4.0.</license-p></license><license xml:lang="en" license-type="creative-commons-attribution" xlink:href="https://creativecommons.org/licenses/by/4.0/" xlink:type="simple"><license-p>This work is licensed under a Creative Commons Attribution 4.0 License.</license-p></license></permissions><self-uri xlink:href="https://www.mrsej.ru/jour/article/view/222">https://www.mrsej.ru/jour/article/view/222</self-uri><abstract><p>We use transformation properties of the irreducible representations of the symmetry group of the Hamiltonian and properties of a continuous path to define a "failure tree" procedure for finding eigenvalues of the Schrodinger equation using stochastic methods. The procedure is used to calculate energies of the lowest excited states of quantum systems possessing anti-symmetric nodal regions in configuration space with the Feynman-Kac path integral method. Within this method, the solution of the imaginary time Schrodinger equation is approximated by random walk simulations on a discrete grid constrained only by symmetry considerations of the Hamiltonian. The required symmetry constraints on random walk simulations are associated with a given irreducible representation of a subgroup of the symmetry group of the Hamiltonian and are found by identifying the eigenvalues for the irreducible representation corresponding to symmetric or antisymmetric eigenfunctions for each group operator. As a consequence, the sign problem for fermions is eliminated. The method provides exact eigenvalues of excited states in the limit of infinitesimal step size and infinite time. The numerical method is applied to compute the eigenvalues of the lowest excited states of the hydrogenic and helium atoms.</p></abstract><funding-group><funding-statement xml:lang="en">This work is dedicated to Boris I. Kochelaev on the occasion of his 70th birthday and supported in part by the Robert A. Welch Foundation.</funding-statement></funding-group></article-meta></front><back><ref-list><title>References</title><ref id="cit1"><label>1</label><citation-alternatives><mixed-citation xml:lang="ru">P. Exner, Open Quantum Systems and Feynman Integrals (Reidel Pub. Co., Boston, 1985) Chapter 5, p. 217.</mixed-citation><mixed-citation xml:lang="en">P. Exner, Open Quantum Systems and Feynman Integrals (Reidel Pub. Co., Boston, 1985) Chapter 5, p. 217.</mixed-citation></citation-alternatives></ref><ref id="cit2"><label>2</label><citation-alternatives><mixed-citation xml:lang="ru">M. Reed and B. 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