Including van der Waals interactions in first-principles electronic structure calculations

Dispersion (van der Waals or vdW) interactions are long-range, non-local in nature, and can be important for understanding and predicting structure and energetics in many systems. Examples of such systems include weakly bound dimers, molecules on surfaces and molecular crystals. Because of the inherent non-locality of these interactions, they are not accounted for by traditional local and semi-local exchange and correlation functionals in density functional theory (DFT). In this thesis, two different approaches to including dispersion interactions in DFT were investigated and implemented. The first approach is based on a recently developed method [Silvestrelli, PRL 100, 053002 (2008)] that maps the DFT ground-state electron density onto a set of maximally-localized Wannier functions (MLWFs). These MLWFs act as fragments of electron density that are used in a pairwise summation of the vdW contribution to the total energy. This contribution is added to the total DFT ground-state energy in a post-processing fashion. The method, as originally proposed, has a number of shortcomings that hamper its predictive power. To overcome these problems, we developed and implemented a number of improvements to it and demonstrated that these modifications give rise to calculated binding energies and equilibrium geometries that are in closer agreement to results of quantum-chemical coupled-cluster calculations. The second approach, known as the vdW density functional (vdW-DF) method, incorporates a non-local vdW term directly into the exchange and correlation functional. Following a recent efficient implementation [Guillermo Román-Pérez and José M. Soler, PRL 103, 096102 (2009)] we coded this approach and a number of vdW functionals (vdW-DF, vdW-DF2, optB88, optPBE) in the ONETEP linear-scaling DFT package, enabling treatment of very large systems that were previously too computationally demanding for such methods. We applied the vdW-DF method to a system of interest for applications in photovoltaics, namely fullerene (C60) molecular crystals, and investigated the effect of including vdW interactions on the relative stability of different crystal structures.Open Acces

Van der Waals interactions in DFT made easy by Wannier functions

Ubiquitous Van der Waals interactions between atoms and molecules are important for many molecular and solid structures. These systems are often studied from first principles using the Density Functional Theory (DFT). However, the commonly used DFT functionals fail to capture the essence of Van der Waals effects. Many attempts to correct for this problem have been proposed, which are not completely satisfactory because they are either very complex and computationally expensive or have a basic semiempirical character. We here describe a novel approach, based on the use of the Maximally-Localized Wannier functions, that appears to be promising, being simple, efficient, accurate, and transferable (charge polarization effects are naturally included). The results of test applications are presented.Comment: submitted to Phys. Rev. Let

Sex and Sexual Orientation Differences in Perceptual and Cognitive Processing

Sex differences have been found in some visual perception and cognitive abilities, and male and female brains have been shown to have differences in functional activation. These abilities include mental rotation, face recognition and face detection. One way to measure mental rotation is by using the mental rotation task (MRT), with males outperforming females (Voyer, 2011). Face perception tasks show differences favouring females (McBain et al., 2009; Brewster et al., 2012). Same-sex attracted males tend to perform at the level of females in face recognition ability (Brewster et al., 2012). This thesis seeks to further examine the effect sexual orientation has on these visual and perceptual abilities that have previously shown sex differences. A male advantage was found for mental rotation ability, with heterosexual males outperforming heterosexual females. Within the same-sex attracted groups, this difference was not found, with same-sex attracted females performing at the level of same-sex attracted males

Strategies and outcomes of HIV status disclosure in HIV-positive young women with abuse histories

Young women with HIV and histories of physical and/or sexual abuse in childhood may be vulnerable to difficulties with disclosure to sexual partners. Abuse in childhood is highly prevalent in HIV-positive women, and has been associated with poorer communication, low assertiveness, low self worth, and increased risk for sexual and other risk behaviors that increase the risk of secondary transmission of HIV. HIV disclosure may be an important link between abuse and sexual risk behaviors. Qualitative interviews with 40 HIV-positive young women with childhood physical and/or sexual abuse were conducted; some women had also experienced adult victimization. Results suggest that HIV-positive women with abuse histories use a host of strategies to deal with disclosure of HIV status, including delaying disclosure, assessing hypothetical responses of partners, and determining appropriate stages in a relationship to disclose. Stigma was an important theme related to disclosure. We discuss how these disclosure processes impact sexual behavior and relationships and discuss intervention opportunities based on our findings

The potential of imogolite nanotubes as (co-)photocatalysts : a linear-scaling density functional theory study

We report a linear-scaling density functional theory (DFT) study of the structure, wall-polarization absolute band-alignment and optical absorption of several, recently synthesized, open-ended imogolite (Imo) nanotubes (NTs), namely single-walled (SW) aluminosilicate (AlSi), SW aluminogermanate (AlGe), SW methylated aluminosilicate (AlSi-Me), and double-walled (DW) AlGe NTs. Simulations with three different semi-local and dispersion-corrected DFT-functionals reveal that the NT wall-polarization can be increased by nearly a factor of four going from SW-AlSi-Me to DW-AlGe. Absolute vacuum alignment of the NT electronic bands and comparison with those of rutile and anatase TiO2 suggest that the NTs may exhibit marked propensity to both photo-reduction and hole-scavenging. Characterization of the NTs' band-separation and optical properties reveal the occurrence of (near-)UV inside–outside charge-transfer excitations, which may be effective for electron–hole separation and enhanced photocatalytic activity. Finally, the effects of the NTs' wall-polarization on the absolute alignment of electron and hole acceptor states of interacting water (H2O) molecules are quantified and discussed

Chemically selective alternatives to photoferroelectrics for polarization-enhanced photocatalysis: the untapped potential of hybrid inorganic nanotubes

Linear-scaling density functional theory simulation of methylated imogolite nanotubes (NTs) elucidates the interplay between wall-polarization, bands separation, charge-transfer excitation, and tunable electrostatics inside and outside the NT-cavity. The results suggest that integration of polarization-enhanced selective photocatalysis and chemical separation into one overall dipole-free material should be possible. Strategies are proposed to increase the NT polarization for maximally enhanced electron–hole separation

Community end user perceptions of hessian fabric transfluthrin vapour emanators for protecting against mosquitoes under conditions of routine use in Port-au-Prince, Haiti

Background: A treated fabric device for emanating the volatile pyrethroid transfluthrin was recently developed in Tanzania that protected against night-biting Anopheles and Culex mosquitoes for several months. Here perceptions of community end users provided with such transfluthrin emanators, primarily intended to protect them against day-active Aedes vectors of human arboviruses that often attack people outdoors, were assessed in Port-au-Prince, Haiti. Methods: Following the distribution of transfluthrin emanators to participating households in poor-to-middle class urban neighbourhoods, questionnaire surveys and in-depth interviews of end-user households were supplemented with conventional and Photovoice-based focus group discussions. Observations were assessed synthetically to evaluate user perceptions of protection and acceptability, and to solicit advice for improving and promoting them in the future. Results: Many participants viewed emanators positively and several outlined various advantages over current alternatives, although some expressed concerns about smell, health hazards, bulkiness, unattractiveness and future cost. Most participants expressed moderate to high satisfaction with protection against mosquitoes, especially indoors. Protection against other arthropod pests was also commonly reported, although satisfaction levels were highly variable. Diverse use practices were reported, some of which probably targeted nocturnal Culex resting indoors, rather than Aedes attacking them outdoors during daylight hours. Perceived durability of protection varied: While many participants noted some slow loss over months, others noted rapid decline within days. A few participants specifically attributed efficacy loss to outdoor use and exposure to wind or moisture. Many expressed stringent expectations of satisfactory protection levels, with even a single mosquito bite considered unsatisfactory. Some participants considered emanators superior to fans, bedsheets, sprays and coils, but it is concerning that several preferred them to bed nets and consequently stopped using the latter. Conclusions: The perspectives shared by Haitian end-users are consistent with those from similar studies in Brazil and recent epidemiological evidence from Peru that other transfluthrin emanator products can protect against arbovirus infection. While these encouraging sociological observations contrast starkly with evidence of essentially negligible effects upon Aedes landing rates from parallel entomological assessments across Haiti, Tanzania, Brazil and Peru, no other reason to doubt the generally encouraging views expressed herein by Haitian end users could be identified

Risky sexual practices among youth attending a sexually transmitted infection clinic in Dar es Salaam, Tanzania

\ud Youth have been reported to be at a higher risk of acquiring STIs with significant adverse health and social consequences. Knowledge on the prevailing risky practices is an essential tool to guide preventive strategies. Youth aged between 18 and 25 years attending an STI clinic were recruited. Social, sexual and demographic characteristics were elicited using a structured standard questionnaire. Blood samples were tested for syphilis and HIV infections. Urethral, high vaginal and cervical swabs were screened for common STI agents. A total of 304 youth were studied with mean age of 21.5 and 20.3 years for males and females respectively. 63.5% of youth were seeking STI care. The mean age of coitache was 16.4 and 16.2 years for males and females respectively. The first sexual partner was significantly older in females compared to male youth (23.0 vs 16.8 years) (p < 0.01). 93.2% of male youth reported more than one sexual lifetime partner compared to 63.0% of the females. Only 50% of males compared to 43% of females had ever used a condom and fewer than 8.3% of female youth used other contraceptive methods. 27.1% of pregnancies were unplanned and 60% of abortions were induced. 42.0% of female youth had received gifts/money for sexual favours. The HIV prevalence was 15.3% and 7.5% for females and males respectively. The prevalence of other STIs was relatively low. Among male youth, use of alcohol or illicit drugs was associated with increased risk of HIV infection. However, the age of sexual initiation, number of sexual partners or the age of the first sexual partner were not associated with increased risk of being HIV infected. Most female youth seen at the STI clinic had their first sexual intercourse with older males. Youth were engaging in high risk unprotected sexual practices which were predisposing them to STIs and unplanned pregnancies. There is a great need to establish more youth-friendly reproductive health clinics, encourage consistent and correct use of condoms, delay in sexual debut and avoid older sexual partners in females.\u

Development of a test facility for experimental investigation of fluid-structure interaction

Fluid-structure interaction phenomena are strongly related to the loading appearing on many energy converting components introducing limitations for improving their efficiency. The term “fluid-structure interaction” includes many phenomena with the “shock wave – boundary layer interaction” being one of the most important. This interaction is commonly met in turbomachines where the flow can accelerate enough to become compressible and can cause separation of the boundary layer formed on the structural components of the machine. This results to fluctuating loading on the structure which can lead to its failure due to High Cycle Fatigue (HCF). A vibrating structure in compressible flow can become unstable depending on the sign of the aerodynamic damping that the flow has on the structure. Although the mechanism that causes a structure to become unstable is known, the limits of the stability region are not yet possible to predict with reasonable accuracy. It is therefore necessary to investigate the underlying mechanism of fluid-structure interaction by means of experimental and numerical studies for providing prediction tools regarding the stability change. The present work aims at developing an experimental facility to be used for investigating fluid-structure interaction. The experimental setup is based on the concept of a simplified aeroelastic test case bringing into focus the area of interaction between an oscillating shock wave and a turbulent boundary layer. This work is based on previous research campaigns using the same generic experimental concept but takes the investigation further to higher and so far unexplored reduced frequencies. The experimental setup has been validated regarding its suitability to meet the research objectives by running vibration tests at an initial stage without the effect of flow. The results from the experimental validation of the facility have shown that the design objectives are met. Specifically the vibration response of the test object concerning vibration amplitude and vibration mode shape is desirable; the vibration amplitude is in the range of 0.5mm and the mode shape remains below the 2nd throughout the targeted frequency range (0-250Hz). This makes the facility suitable for simplified investigation of fluid-structure interaction, bringing the shock foot region into focus. Having validated the facility performing vibration tests without flow, tests with flow is the next step to take place. Since the vibration response of the test object has been investigated in detail, tests with flow will reveal the influence of fluidstructure interaction on the dynamic response of the test object. Similarly, the influence of this interaction on the flow side can be assessed by monitoring the flow parameters. As a first step for performing this investigation, the design study and the validation results for the experimental setup are presented in this work.