29 research outputs found
Extension and applications of the GVVPT2 method to the study of transition metals
The ground and low-lying excited electronic states of molecules of the first ( 2 Sc , 2 Cr , 2 Mn , and 2 Ni ) and second ( 2 Y , 2 Mo , and 2 Tc ) row of transition elements have been investigated for the first time with the generalized Van Vleck second order multireference perturbation theory (GVVPT2) method, a variant of MRPT. All potential energy curves (PECs) obtained in these studies were smooth and continuous; that is, they are free from wiggles or inflexion points. In order to account for relativistic effects, which become important in heavy elements, the GVVPT2 method was extended to include scalar relativistic effects through the spin-free exact two component (sf-X2C) method and used in the studies of all molecules of second row transition elements and some of those of the first row considered in this present work. GVVPT2 studies of triatomic lithium and beryllium were also done as a first step to studies of small clusters of transition metals. The spectroscopic constants (bond lengths, harmonic frequencies, bond energies, and adiabatic transition energies) obtained for all PECs at the GVVPT2 level were in good agreement with experimental data, where available, and with results from previous studies using other high level ab initio methods. Optimized geometries of the triatomics were also in good agreement with previous findings. The studies included electronic states (e.g., the g 1 g 1 2 ÎŁ and 3 ÎŁ states of 2 Y as well as the g 5 1 ÎŁ and g 9 1 ÎŁ states of 2 Tc ) not previously discussed in the literature.
As a first step to applying GVVPT2 to the study of relatively larger systems, the present work includes the results of efforts on improving DFT-in-DFT embedding theory. New equations were determined which involved an additional constraint of orthogonality of the orbitals of one subsystem to those of the complementary subsystem as warranted by formal arguments based on the formulation of DFT-in-DFT embedding. A computer program was realized using the new embedding equations and test calculations performed. Analyses of electron density deformations in embedding theory, in comparison with conventional Kohn-Sham (KS)-DFT densities, were performed using the new embedding program and a computer code that was also written to compute electron densities of molecules in real space, given reduced one particle density matrices. The results revealed that whereas the current formulation of DFT-in-DFT embedding theory generally underestimates electron density, at the interface between subsystems in comparison with conventional KS-DFT calculations of the supermolecule, the new DFT-in-DFT embedding scheme with the external orthogonality constraint was found to remedy the situation. Worthy of special note in this new embedding protocol is the fact that the nonadditive kinetic potential ( T v ), thought to be a major cause of weaknesses in DFT-in-DFT embedding and to which many previous research efforts have been devoted, can be set exactly to zero. The present work therefore realized, for the first time, a new DFT-in-DFT embedding theory that neither relies on kinetic functionals nor requires a supermolecular DFT calculation. Test calculations using the new embedding theory and supermolecular basis set expansion of KS orbitals reproduced conventional KS-DFT energies to at least the 7th decimal place (and even exactly at many geometries). A new way of expanding KS orbitals was also employed in the new embedding protocol, which is intermediate between the usual supermolecular and monomer basis expansions, referred to as the âextended monomer expansionâ. The monomer basis expansion scheme was inadequate for the new DFT-in-DFT embedding protocol. Test calculations found this novel, computationally cheaper, extended monomer approach to give results quite close to those from supermolecular basis expansions
Cameroonian Immigrants Switching From Non-Stem To Stem-And-Related Fields In The United States
Recent studies on STEM education in the United States (U.S.) have largely focused on such issues as the ever increasing need for more STEM workers, STEM attrition/retention, the gender gap in STEM, encouraging more STEM enrollment both at college and post-graduate levels, and the use of modern technology to facilitate STEM education. Other studies have considered the impact of immigrants on STEM education in the U.S. and on the U.S. economy in terms of either creating or filling STEM jobs. Many of those studies underscore the importance of immigrants in boosting the high skilled, much needed, STEM labor force and thereby increasing the global competitiveness of the U.S.
Despite a large volume of STEM-related research in the U.S., minimal research has focused on immigrants who enter the U.S. without prior STEM backgrounds and choose STEM or related majors while in the U.S. In particular, no prior study has considered Cameroonian immigrants with non-STEM majors and who switch into STEM or related fields upon arrival in the U.S. Such studies could unveil key factors that serve as strong motivators for STEM enrollment.
To fill this gap, the present study investigated why Cameroonian immigrants who came into the U.S. without a prior STEM background switched into STEM or related fields while in the U.S. The study involved 8 Cameroonian immigrants who were majoring in either a STEM or related field who came into the U.S. without a prior STEM background. Data was obtained through in-depth, semi-structured, face-to-face, individual participant interviews.
Participants attributed their failure to major in STEM while in Cameroon to the following: stereotypes, limited financial and other resources, negative influence from teachers, peers, relatives, and others, and the impact of non-STEM role models. Participants attributed their choice to switch into STEM or related fields while in the U.S. to job discrimination within non-STEM fields, ease of employment within STEM, and the impact of friends and others who directly encouraged them to switch. In comparing STEM education in Cameroon and the U.S., participants felt the U.S. offered many advantages such as increased flexibility, teacher accessibility, and financial and other resources. It was found that although participants had faced challenges upon switching into STEM in the U.S., their determination/resilience had led them to overcome those challenges and persist in their new majors. Overall, gender differences were insignificant in the decision to switch to a STEM or related major in the present study
Towards Better Management of Public Education in Cameroon: The Case for Decentralisation
This paper conceptualises centralisation and decentralisation and provides evidence that the Cameroon public education system is facing imminent decentralisation. It uses available literature and the author's teaching experience to criticise the centralised education system in Cameroon, especially in resource allocation and management. The author argues that to increase the quantity and quality of education in Cameroon, there is a need to decentralise responsibility for and control of education. He presents the model proposed by government for decentralisation and proposes roles and functions for the various levels of authority in the model.
Résumé
Cet article conceptualise les notions de centralisation et de décentralisation et démontre que le systÚme éducatif camerounais va au devant d\'une décentralisation imminente. Il se base sur la littérature disponible à ce sujet, ainsi que sur l\'expérience pédagogique, pour critiquer la centralisation du systÚme éducatif camerounais, particuliÚrement en matiÚre de distribution et de gestion des ressources. L\'auteur avance que, pour améliorer la quantité et la qualité de l\'enseignement au Cameroun, il est indispensable de décentraliser l\'imputation et le contrÎle y afférent. Il présente le modÚle proposé par le gouvernement en vue de la décentralisation prévue, et suggÚre des rÎles et fonctions pour les divers niveaux d\'autorité contenus dans le modÚle.
Africa Development/Afrique et développement Vol.XXIX, No 2, 2004: 134-15
Prevalence and factors associated with drug therapy problems among hypertensive patients at hypertension clinic of Mbarara Regional Referral Hospital, Uganda: a |cross-sectional study
Background: Despite the use of safe and effective conventional drugs, drug therapy problems (DTPs) pose a threat to the successful management of hypertension. DTPs are of a great concern in health care because of their serious consequences such as poor quality of life, increased health care costs, morbidity and mortality. However, there is no published information regarding the prevalence of DTPs and associated factors among hypertensive patients in Uganda. Objective: The aim of the study was to determine the prevalence and factors associated with DTPs among hypertensive patients at the hypertension clinic of Mbarara Regional Referral Hospital (MRRH). Method: A cross-sectional study was conducted at the hypertension clinic, MRRH, Uganda among 228 hypertensive patients. Data were collected from medical records using a data abstraction tool and patients were interviewed using a structured questionnaire. Data analysis was done using Statistical Package for Social Sciences (SPSS) version 22.0. Descriptive analysis was used to determine the prevalence of DTPs. Logistic regression was used to determine the association between the independent and dependent variables. Variables were considered statistically significant at p -value <0.05. Results: A total of 178 DTPs were identified among 141 hypertensive patients. The prevalence of antihypertensive-related DTPs was 61.8% (95% confidence interval [CI]: 55.3â67.5) with an average of 1.26â±â0.52 DTPs per patient. Out of 141 participants with DTPs, 109 (77.3%) had one DTP, 27 (19.1%) had 2 DTPs, and 5 (3.5%) had 3 DTPs. The most common types of antihypertensive-related DTPs were âdosage too lowâ which accounted for 53 (29.8%), followed by âadverse drug reactionsâ which accounted for 48 (27%). Uncontrolled blood pressure (BP; adjusted odds ratio [AOR]: 4.17; 95% CI: 2.33â7.45, p â<â0.001) and routine laboratory test results (AOR: 1.87; 95% CI: 1.04â3.36, p â=â0.036) were significantly associated with antihypertensive-related DTPs among hypertensive patients. Conclusion: Almost two-thirds of study participants had antihypertensive-related DTPs. The most common DTPs were âdosage too lowâ and âadverse drug reactionsâ which both accounted for almost a third of the total DTPs each. Uncontrolled BP and routine laboratory test results were significantly associated with antihypertensive-related DTPs among the study participants. Our study emphasizes the need for improved patient care by clinical pharmacists to identify and prevent DTPs among hypertensive patients
Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency
Semiconductor
quantum dots (QDs) functionalized by metalâorganic
dyes show great promise in photocatalytic and photovoltaic applications.
However, the charge transfer direction and ratesîžkey processes
governing the efficiency of energy conversionîžare strongly
affected by the QDâdye interactions, insights on which are
challenging to obtain experimentally. We use density functional theory
(DFT) and constrained DFT calculations to investigate a degree of
sensitivity of the electronic level alignment and related QDâdye
electronic couplings to binding conformations of N719 dye at the surface
of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations
between the strength of the QDâdye interaction in terms of
their binding conformations and the donorâacceptor electronic
couplings. While the QDâdye binding conformations are the most
stable when the N719 dye is attached to the QD via two carboxylate
groups, the strongest electronic coupling between the QD as an electron
donor and the dye as an electron acceptor is observed in structures
bonded via the isocyanate ligands. Such strong electronic couplings
also are responsible for significant stabilization of the dyeâs
occupied orbitals deep inside in the valence band of the QD making
the hole transfer from the photoexcited QD to the dye thermodynamically
unfavorable in structures bound via isocyanates. Our results suggest
that the most probable binding conformations are those occurring via
two carboxylate linkers, which exhibit very weak electronic couplings
contributing to the electron transfer from the photoexcited CdSe QD
to the N719 dye but provide the most favorable conditions for the
hole transfer. Overall, our computational work provides an insightful
view about the surface chemistry of CdSe regarding the donorâacceptor
interaction, energy level alignment, and charge transfer between CdSe
and dye molecule, which can guide the rational design of QD-based
materials for energy conversion applications
Relativistic GVVPT2 Multireference Perturbation Theory Description of the Electronic States of Y<sub>2</sub> and Tc<sub>2</sub>
The multireference generalized Van
Vleck second-order perturbation theory (GVVPT2) method is used to
describe full potential energy curves (PECs) of low-lying states of
second-row transition metal dimers Y<sub>2</sub> and Tc<sub>2</sub>, with scalar relativity included via the spin-free exact two-component
(sf-X2C) Hamiltonian. Chemically motivated incomplete model spaces,
of the style previously shown to describe complicated first-row transition
metal diatoms well, were used and again shown to be effective. The
studied states include the previously uncharacterized 2<sup>1</sup>ÎŁ<sub>g</sub><sup>+</sup> and
3<sup>1</sup>ÎŁ<sub>g</sub><sup>+</sup> PECs of Y<sub>2</sub>. These states, together with 1<sup>1</sup>ÎŁ<sub>g</sub><sup>+</sup>, are relevant to discussion of controversial results in the literature
that suggest dissociation asymptotes that violate the noncrossing
rule. The ground state of Y<sub>2</sub> was found to be X<sup>5</sup>ÎŁ<sub>u</sub><sup>â</sup> (similar to Sc<sub>2</sub>) with bond length <i>R</i><sub>e</sub> = 2.80 Ă
, binding energy <i>D</i><sub>e</sub> = 3.12 eV, and harmonic frequency Ï<sub>e</sub> = 287.2 cm<sup>â1</sup>, whereas the lowest 1<sup>1</sup>ÎŁ<sub>g</sub><sup>+</sup> state of Y<sub>2</sub> was found to lie 0.67 eV above the quintet ground state and
had spectroscopic constants <i>R</i><sub>e</sub> = 3.21
Ă
, <i>D</i><sub>e</sub> = 0.91 eV, and Ï<sub>e</sub> = 140.0 cm<sup>â1</sup>. Calculations performed on
Tc<sub>2</sub> include study of the previously uncharacterized relatively
low-lying 1<sup>5</sup>ÎŁ<sub>g</sub><sup>+</sup> and 1<sup>9</sup>ÎŁ<sub>g</sub><sup>+</sup> states (i.e., 0.70 and 1.84 eV
above 1<sup>1</sup>ÎŁ<sub>g</sub><sup>+</sup>, respectively). The ground state of Tc<sub>2</sub> was found to be X<sup>3</sup>ÎŁ<sub>g</sub><sup>â</sup> with <i>R</i><sub>e</sub> = 2.13 Ă
, <i>D</i><sub>e</sub> = 3.50
eV, and Ï<sub>e</sub> = 336.6 cm<sup>â1</sup> (for the
most stable isotope, Tc-98) whereas the lowest <sup>1</sup>ÎŁ<sub>g</sub><sup>+</sup> state, generally
accepted to be the ground state symmetry for isovalent Mn<sub>2</sub> and Re<sub>2</sub>, was found to lie 0.47 eV above the X<sup>3</sup>ÎŁ<sub>g</sub><sup>â</sup> state of Tc<sub>2</sub>. The results broaden the range of demonstrated
applicability of the GVVPT2 method